# Longitudinal Growth Curves

> Use when tracking a user's evolution over time through activity stages, modeling temporal trajectories of engagement or participation, detecting phase transitions in longitudinal behavioral data, classifying digital maturity stages from activity metadata, or comparing growth curve model fits on time-series activity data

- Skill: `aaddrick/longitudinal-growth-curves` (Agent Skill)
- Install (CLI): `npx skillmds@latest add aaddrick/longitudinal-growth-curves`
- Raw SKILL.md: https://api.skillmd.com/api/skills/aaddrick/longitudinal-growth-curves/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Marketing & Growth
- Author: aaddrick (https://skillmd.com/u/aaddrick)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/aaddrick/longitudinal-growth-curves

---


# Longitudinal Growth Curve Modeling

## Overview

Fit growth curve models to temporal activity metadata to track a user's evolution through stages of Digital Maturity (Lurker, Participant, Community Leader/Moderator). The core principle: **activity trajectories are not linear -- fit multiple candidate models, use information criteria to select the best, and detect change points that mark stage transitions rather than assuming smooth progression.**

## When to Use

- Temporal activity data spanning months or years (timestamps on posts, comments, votes, moderation actions)
- Need to classify a user's current Digital Maturity stage based on trajectory shape
- Investigating whether a user transitioned between participation modes (lurking to active, active to leadership)
- Comparing growth models (linear, logistic, piecewise) to find best trajectory fit
- Detecting inflection points or change points in activity rate over time
- Feeding trajectory shape into downstream persona or archetype analysis

**When NOT to use:**
- Activity timeline is shorter than 3 months -- insufficient for trajectory modeling (see Insufficient Data)
- Fewer than 12 time-windowed data points -- cannot distinguish model shapes
- Need to predict future behavior -- this skill models historical trajectories, not forecasts
- Claiming psychological maturity from activity counts -- activity volume is a proxy, not a direct measure
- Single-point-in-time snapshots with no temporal dimension

```dot
digraph model_selection {
    rankdir=LR;
    "Activity trajectory\nshape?" [shape=diamond];
    "Steady increase\nor decrease?" [shape=diamond];
    "Plateau or\nsaturation?" [shape=diamond];
    "Abrupt shifts?" [shape=diamond];

    "Linear model" [shape=box];
    "Logistic model" [shape=box];
    "Piecewise model" [shape=box];
    "Fit ALL three\nand compare AIC/BIC" [shape=box, style=filled, fillcolor="#ccffcc"];

    "Activity trajectory\nshape?" -> "Steady increase\nor decrease?" [label="visual check"];
    "Steady increase\nor decrease?" -> "Linear model" [label="yes"];
    "Steady increase\nor decrease?" -> "Plateau or\nsaturation?" [label="no"];
    "Plateau or\nsaturation?" -> "Logistic model" [label="yes"];
    "Plateau or\nsaturation?" -> "Abrupt shifts?" [label="no"];
    "Abrupt shifts?" -> "Piecewise model" [label="yes"];

    "Linear model" -> "Fit ALL three\nand compare AIC/BIC";
    "Logistic model" -> "Fit ALL three\nand compare AIC/BIC";
    "Piecewise model" -> "Fit ALL three\nand compare AIC/BIC";
}
```

**Always fit all candidate models regardless of visual impression.** The flowchart above guides intuition, but model selection MUST be data-driven via AIC/BIC comparison.

## Quick Reference

| Model | When It Fits | Parameters | Best For |
|-------|-------------|------------|----------|
| **Linear** | Steady, constant-rate change | slope, intercept | Early-stage users with short timelines |
| **Logistic (sigmoid)** | S-curve: slow start, rapid growth, plateau | L (ceiling), k (steepness), x0 (midpoint) | Users who ramp up then stabilize |
| **Piecewise linear** | Distinct phases with different slopes | slopes per segment, breakpoints | Users with clear stage transitions |
| **Quadratic/polynomial** | Gradual acceleration or deceleration | coefficients | Use sparingly; overfits easily beyond degree 2 |

| Information Criterion | Formula Concept | Use Case |
|-----------------------|-----------------|----------|
| **AIC** | -2*log(L) + 2*k | Prefer for predictive model selection |
| **BIC** | -2*log(L) + k*log(n) | Prefer for explanatory model selection (penalizes complexity more) |

**Selection rule:** Lower AIC/BIC = better fit. When AIC and BIC disagree, prefer BIC (we seek explanation, not prediction).

## Digital Maturity Stages

| Stage | Label | Behavioral Signals |
|-------|-------|-------------------|
| **Stage 1** | Lurker | Low or zero posting activity; account exists but minimal visible output; may have passive signals (subscriptions, saves, votes) if available |
| **Stage 2** | Participant | Regular posting/commenting; engaged in discussions; moderate frequency and consistency |
| **Stage 3** | Community Leader/Moderator | High-frequency sustained activity; moderation actions; thread initiation; mentoring behaviors; cross-community engagement |

**Transition detection is the primary analytical goal** -- not just classifying current stage but identifying WHEN and HOW transitions occurred.

## Workflow

Copy this checklist and track progress:

```
Longitudinal Growth Curve Modeling Progress:
- [ ] Step 1: Prepare temporal activity data (windowed aggregation)
- [ ] Step 2: Compute activity features per time window
- [ ] Step 3: Visualize raw trajectory (before fitting)
- [ ] Step 4: Fit linear growth model
- [ ] Step 5: Fit logistic (sigmoid) growth model
- [ ] Step 6: Fit piecewise linear model with change point detection
- [ ] Step 7: Compare models using AIC/BIC
- [ ] Step 8: Detect phase transitions (change points)
- [ ] Step 9: Classify Digital Maturity stages
- [ ] Step 10: Validate transitions against activity metadata
- [ ] Step 11: Write findings to docs/analysis/11-longitudinal-growth-curves.md
```

### Step 1: Prepare Temporal Activity Data (Windowed Aggregation)

Raw timestamps must be aggregated into time windows before fitting. Do NOT fit models to individual event timestamps.

```python
import pandas as pd
import numpy as np

def prepare_temporal_windows(activity_df, date_col='date', window='W'):
    """
    Aggregate activity into regular time windows.

    Adapt to your data:
    - Forum data: post/comment timestamps
    - Chat data: message timestamps
    - Any platform: action timestamps with action_type column

    Parameters:
    - activity_df: DataFrame with at least a datetime column
    - date_col: name of the datetime column
    - window: pandas frequency string ('W'=weekly, 'M'=monthly, '2W'=biweekly)

    Returns:
    - DataFrame with one row per time window containing activity counts
    """
    df = activity_df.copy()
    df[date_col] = pd.to_datetime(df[date_col])

    # Sort by date for proper windowing
    df = df.sort_values(date_col)

    # Determine appropriate window size based on timeline span
    timeline_span = (df[date_col].max() - df[date_col].min()).days
    if window == 'auto':
        if timeline_span < 90:
            window = 'W'       # Weekly for short timelines
        elif timeline_span < 365:
            window = '2W'      # Biweekly for medium timelines
        else:
            window = 'M'       # Monthly for long timelines

    # Create a complete date range (fills gaps with zero-activity windows)
    full_range = pd.date_range(
        start=df[date_col].min(),
        end=df[date_col].max(),
        freq=window
    )

    # Aggregate activity per window
    windowed = df.set_index(date_col).resample(window).agg(
        activity_count=('body' if 'body' in df.columns else date_col, 'count'),
    ).reindex(full_range, fill_value=0)

    windowed.index.name = 'window_start'
    windowed = windowed.reset_index()

    # Add numeric time index for regression (0, 1, 2, ...)
    windowed['t'] = range(len(windowed))

    print(f"Timeline: {df[date_col].min().date()} to {df[date_col].max().date()}")
    print(f"Window size: {window}, Total windows: {len(windowed)}")
    print(f"Windows with zero activity: {(windowed['activity_count'] == 0).sum()}")

    return windowed
```

**Critical: fill zero-activity windows.** Gaps in activity ARE data -- they indicate lurking or absence. Dropping zero-activity windows biases the trajectory upward.

**Window size selection:**

| Timeline Span | Recommended Window | Rationale |
|---------------|-------------------|-----------|
| < 3 months | Weekly | Preserves short-term variation |
| 3-12 months | Biweekly | Smooths noise while preserving trends |
| 1-3 years | Monthly | Standard for long-term trajectories |
| 3+ years | Monthly | Monthly is sufficient; quarterly loses too much detail |

### Step 2: Compute Activity Features Per Time Window

Activity volume alone is an insufficient maturity indicator. Compute multiple features per window.

```python
def compute_window_features(activity_df, date_col='date', window='M'):
    """
    Compute multi-dimensional activity features per time window.

    Adapt feature computation to available columns in your data:
    - If you have post types: count by type (post vs comment vs moderation)
    - If you have engagement scores: aggregate per window
    - If you have reply chains: count thread initiations vs replies
    """
    df = activity_df.copy()
    df[date_col] = pd.to_datetime(df[date_col])

    features = df.set_index(date_col).resample(window).agg(
        activity_count=(date_col, 'count'),
    )

    # Add content-length feature if text body is available
    if 'body' in df.columns:
        body_agg = df.set_index(date_col)['body'].resample(window).agg(
            lambda x: x.dropna().str.len().mean()
        )
        features['avg_content_length'] = body_agg

    # Add unique-context feature if context/subreddit/channel is available
    context_col = next(
        (c for c in ['subreddit', 'channel', 'forum', 'category']
         if c in df.columns), None
    )
    if context_col:
        context_agg = df.set_index(date_col)[context_col].resample(window).nunique()
        features['unique_contexts'] = context_agg

    # Add content-type diversity if type column exists
    type_col = next(
        (c for c in ['type', 'content_type', 'action_type']
         if c in df.columns), None
    )
    if type_col:
        type_agg = df.set_index(date_col)[type_col].resample(window).nunique()
        features['action_diversity'] = type_agg

    # Fill gaps with zeros for count features, NaN for ratio features
    features['activity_count'] = features['activity_count'].fillna(0)

    features = features.reset_index()
    features.columns.name = None
    features['t'] = range(len(features))

    return features
```

**Multi-dimensional maturity signals:**

| Feature | Lurker Signal | Participant Signal | Leader Signal |
|---------|--------------|-------------------|---------------|
| activity_count | 0-1 per window | Moderate, consistent | High, sustained |
| avg_content_length | N/A or very short | Moderate (50-200 chars) | Long, substantive (200+) |
| unique_contexts | 0-1 | 2-5 | 5+ (cross-community presence) |
| action_diversity | 1 (view/vote only) | 2-3 (post, comment) | 3+ (post, comment, moderate, initiate) |

### Step 3: Visualize Raw Trajectory (Before Fitting)

Always plot the raw data before fitting any model. This prevents fitting a model that contradicts visual evidence.

```python
import matplotlib
matplotlib.use('Agg')
import matplotlib.pyplot as plt

def plot_raw_trajectory(windowed_df, y_col='activity_count', save_path=None):
    """Plot raw activity trajectory with a LOESS smoother for visual trend."""
    fig, ax = plt.subplots(figsize=(12, 5))

    ax.bar(windowed_df['window_start'], windowed_df[y_col],
           alpha=0.4, color='steelblue', label='Raw activity')

    # Rolling average as visual smoother
    rolling_window = max(3, len(windowed_df) // 10)
    smoothed = windowed_df[y_col].rolling(
        window=rolling_window, center=True, min_periods=1
    ).mean()
    ax.plot(windowed_df['window_start'], smoothed,
            color='darkred', linewidth=2, label=f'Rolling mean (w={rolling_window})')

    ax.set_xlabel('Time')
    ax.set_ylabel(y_col)
    ax.set_title(f'Raw Activity Trajectory: {y_col}')
    ax.legend()

    if save_path:
        fig.savefig(save_path, dpi=150, bbox_inches='tight')
        print(f"Saved trajectory plot to {save_path}")
    plt.close(fig)
    return fig
```

**What to look for:**
- Upward trend with steady slope -> likely linear
- S-curve (slow, fast, plateau) -> likely logistic
- Abrupt slope changes -> likely piecewise
- High variance with no trend -> may not be modelable (see Insufficient Data)

### Step 4: Fit Linear Growth Model

```python
import statsmodels.api as sm

def fit_linear_model(windowed_df, y_col='activity_count'):
    """
    Fit a simple linear growth model: y = a + b*t
    Returns model results with AIC/BIC for comparison.
    """
    t = windowed_df['t'].values
    y = windowed_df[y_col].values

    X = sm.add_constant(t)  # Adds intercept column
    model = sm.OLS(y, X).fit()

    return {
        'model_type': 'linear',
        'model': model,
        'aic': model.aic,
        'bic': model.bic,
        'r_squared': model.rsquared,
        'params': {
            'intercept': model.params[0],
            'slope': model.params[1],
        },
        'slope_pvalue': model.pvalues[1],
        'residuals': model.resid,
        'fitted': model.fittedvalues,
        'interpretation': (
            f"Linear trend: {'increasing' if model.params[1] > 0 else 'decreasing'} "
            f"at {model.params[1]:.3f} units/window "
            f"(p={model.pvalues[1]:.4f}, R2={model.rsquared:.3f})"
        ),
    }
```

### Step 5: Fit Logistic (Sigmoid) Growth Model

The logistic model captures S-curve trajectories: slow start, rapid growth, and plateau -- the classic "lurker to active participant who stabilizes" pattern.

```python
from scipy.optimize import curve_fit
from scipy.stats import norm

def logistic_function(t, L, k, t0):
    """
    Logistic growth: y = L / (1 + exp(-k * (t - t0)))
    L = carrying capacity (plateau)
    k = steepness of growth
    t0 = midpoint (inflection point)
    """
    return L / (1 + np.exp(-k * (t - t0)))

def fit_logistic_model(windowed_df, y_col='activity_count'):
    """
    Fit a logistic growth model using nonlinear least squares.
    Returns model results with AIC/BIC for comparison.
    """
    t = windowed_df['t'].values.astype(float)
    y = windowed_df[y_col].values.astype(float)
    n = len(y)

    # Initial parameter guesses
    L_init = y.max() * 1.2 if y.max() > 0 else 1.0
    k_init = 0.1
    t0_init = len(t) / 2

    try:
        popt, pcov = curve_fit(
            logistic_function, t, y,
            p0=[L_init, k_init, t0_init],
            maxfev=10000,
            bounds=([0, 0.001, 0], [y.max() * 5, 10, len(t) * 2])
        )
        L, k, t0 = popt

        fitted = logistic_function(t, *popt)
        residuals = y - fitted
        ss_res = np.sum(residuals**2)
        ss_tot = np.sum((y - y.mean())**2)
        r_squared = 1 - (ss_res / ss_tot) if ss_tot > 0 else 0

        # Compute AIC/BIC for nonlinear model
        # k_params = 3 (L, k, t0) + 1 (sigma) = 4
        k_params = 4
        log_likelihood = -n/2 * np.log(2 * np.pi * ss_res / n) - n/2
        aic = -2 * log_likelihood + 2 * k_params
        bic = -2 * log_likelihood + k_params * np.log(n)

        return {
            'model_type': 'logistic',
            'params': {'L': L, 'k': k, 't0': t0},
            'aic': aic,
            'bic': bic,
            'r_squared': r_squared,
            'residuals': residuals,
            'fitted': fitted,
            'inflection_point': t0,
            'interpretation': (
                f"Logistic fit: ceiling={L:.1f}, steepness={k:.3f}, "
                f"inflection at window {t0:.1f} (R2={r_squared:.3f})"
            ),
            'converged': True,
        }
    except (RuntimeError, ValueError) as e:
        return {
            'model_type': 'logistic',
            'converged': False,
            'error': str(e),
            'interpretation': f"Logistic model failed to converge: {e}",
        }
```

### Step 6: Fit Piecewise Linear Model with Change Point Detection

Piecewise models are the most important for Digital Maturity analysis because stage transitions manifest as slope changes.

```python
def fit_piecewise_model(windowed_df, y_col='activity_count',
                        max_breakpoints=3):
    """
    Fit piecewise linear model using change point detection.

    Uses exhaustive search over candidate breakpoints with BIC
    for selecting the number of segments.

    For large datasets, consider using the 'ruptures' library:
        import ruptures
        algo = ruptures.Pelt(model="l2").fit(signal)
        breakpoints = algo.predict(pen=penalty)
    """
    t = windowed_df['t'].values.astype(float)
    y = windowed_df[y_col].values.astype(float)
    n = len(y)

    if n < 12:
        return {
            'model_type': 'piecewise',
            'converged': False,
            'error': 'Insufficient data points for piecewise fitting (need 12+)',
        }

    best_result = None
    best_bic = np.inf

    # Try 1, 2, ... max_breakpoints
    for n_bp in range(1, max_breakpoints + 1):
        # Candidate breakpoint positions (avoid edges)
        margin = max(3, n // (n_bp + 2))
        candidates = range(margin, n - margin)

        if n_bp == 1:
            for bp in candidates:
                result = _fit_piecewise_segments(t, y, [bp])
                if result['bic'] < best_bic:
                    best_bic = result['bic']
                    best_result = result
        elif n_bp == 2:
            for bp1 in candidates:
                for bp2 in range(bp1 + margin, n - margin):
                    result = _fit_piecewise_segments(t, y, [bp1, bp2])
                    if result['bic'] < best_bic:
                        best_bic = result['bic']
                        best_result = result

    # Also compare against no-breakpoint (simple linear)
    linear_result = _fit_piecewise_segments(t, y, [])
    if linear_result['bic'] < best_bic:
        best_result = linear_result
        best_result['interpretation'] = (
            "No change points detected; linear model preferred by BIC"
        )

    return best_result

def _fit_piecewise_segments(t, y, breakpoints):
    """Fit linear segments between breakpoints and compute BIC."""
    n = len(y)
    segments = []
    boundaries = [0] + list(breakpoints) + [n]

    fitted = np.zeros(n)
    total_residuals = np.zeros(n)

    for i in range(len(boundaries) - 1):
        start, end = boundaries[i], boundaries[i + 1]
        seg_t = t[start:end]
        seg_y = y[start:end]

        if len(seg_t) < 2:
            continue

        X_seg = sm.add_constant(seg_t)
        seg_model = sm.OLS(seg_y, X_seg).fit()
        fitted[start:end] = seg_model.fittedvalues
        total_residuals[start:end] = seg_model.resid

        segments.append({
            'start_idx': start,
            'end_idx': end,
            'slope': seg_model.params[1],
            'intercept': seg_model.params[0],
            'r_squared': seg_model.rsquared,
        })

    ss_res = np.sum(total_residuals**2)
    ss_tot = np.sum((y - y.mean())**2)
    r_squared = 1 - (ss_res / ss_tot) if ss_tot > 0 else 0

    # BIC: k = 2 params per segment + number of breakpoints + sigma
    k_params = 2 * len(segments) + len(breakpoints) + 1
    log_likelihood = -n/2 * np.log(2 * np.pi * ss_res / n) - n/2 if ss_res > 0 else -np.inf
    bic = -2 * log_likelihood + k_params * np.log(n)
    aic = -2 * log_likelihood + 2 * k_params

    return {
        'model_type': 'piecewise',
        'converged': True,
        'breakpoints': breakpoints,
        'segments': segments,
        'aic': aic,
        'bic': bic,
        'r_squared': r_squared,
        'residuals': total_residuals,
        'fitted': fitted,
        'interpretation': (
            f"Piecewise linear: {len(segments)} segments, "
            f"breakpoints at windows {breakpoints} (R2={r_squared:.3f})"
        ),
    }
```

**Alternative: using the `ruptures` library for change point detection:**

```python
def detect_changepoints_ruptures(y, penalty=None, max_breakpoints=5):
    """
    Use the ruptures library for robust change point detection.
    Install: pip install ruptures

    Algorithms:
    - Pelt: penalized, fast, good default (use pen= parameter)
    - Binseg: binary segmentation, fast approximation
    - BottomUp: merges segments, good for noisy data
    """
    import ruptures as rpt

    signal = y.reshape(-1, 1) if y.ndim == 1 else y

    if penalty is None:
        # Use BIC-derived penalty: log(n) * variance
        penalty = np.log(len(y)) * np.var(y)

    algo = rpt.Pelt(model="l2", min_size=3).fit(signal)
    breakpoints = algo.predict(pen=penalty)

    # ruptures returns breakpoint indices (exclusive end positions)
    # Remove the last element (which is always len(signal))
    breakpoints = [bp for bp in breakpoints if bp < len(y)]

    return breakpoints
```

### Step 7: Compare Models Using AIC/BIC

```python
def compare_models(model_results):
    """
    Compare all fitted models using AIC and BIC.
    Returns ranked comparison table.
    """
    comparison = []
    for result in model_results:
        if result.get('converged', True):
            comparison.append({
                'model_type': result['model_type'],
                'aic': result.get('aic', np.inf),
                'bic': result.get('bic', np.inf),
                'r_squared': result.get('r_squared', 0),
                'interpretation': result.get('interpretation', ''),
            })
        else:
            comparison.append({
                'model_type': result['model_type'],
                'aic': np.inf,
                'bic': np.inf,
                'r_squared': 0,
                'interpretation': f"FAILED: {result.get('error', 'did not converge')}",
            })

    comp_df = pd.DataFrame(comparison)

    # Rank by BIC (prefer explanatory parsimony)
    comp_df = comp_df.sort_values('bic')
    comp_df['bic_rank'] = range(1, len(comp_df) + 1)
    comp_df['aic_rank'] = comp_df['aic'].rank().astype(int)

    # Delta BIC from best model
    best_bic = comp_df['bic'].min()
    comp_df['delta_bic'] = comp_df['bic'] - best_bic

    print("\n=== Model Comparison ===")
    print(comp_df[['model_type', 'aic', 'bic', 'delta_bic',
                    'r_squared', 'bic_rank']].to_string(index=False))
    print(f"\nBest model by BIC: {comp_df.iloc[0]['model_type']}")

    return comp_df
```

**Delta BIC interpretation:**

| Delta BIC | Evidence Against Higher-BIC Model |
|-----------|----------------------------------|
| 0-2 | Not worth mentioning; models are essentially equivalent |
| 2-6 | Positive evidence for the lower-BIC model |
| 6-10 | Strong evidence |
| > 10 | Very strong evidence; higher-BIC model is clearly inferior |

**Document your model selection rationale.** State which model won, by how much, and whether AIC and BIC agreed.

### Step 8: Detect Phase Transitions (Change Points)

Whether piecewise wins or not, explicitly detect and characterize transitions.

```python
def detect_phase_transitions(windowed_df, y_col='activity_count',
                              piecewise_result=None):
    """
    Identify and characterize phase transitions in the activity trajectory.
    Uses the piecewise model breakpoints plus slope-change analysis.
    """
    transitions = []

    if piecewise_result and piecewise_result.get('converged'):
        segments = piecewise_result['segments']
        breakpoints = piecewise_result['breakpoints']

        for i, bp in enumerate(breakpoints):
            if i + 1 < len(segments):
                pre_slope = segments[i]['slope']
                post_slope = segments[i + 1]['slope']
                slope_change = post_slope - pre_slope

                # Map breakpoint index to actual date
                bp_date = windowed_df.iloc[bp]['window_start'] \
                    if bp < len(windowed_df) else None

                transitions.append({
                    'breakpoint_index': bp,
                    'date': bp_date,
                    'pre_slope': pre_slope,
                    'post_slope': post_slope,
                    'slope_change': slope_change,
                    'direction': 'acceleration' if slope_change > 0
                                 else 'deceleration',
                    'magnitude': abs(slope_change),
                })

    # Also detect using rolling slope changes (model-free)
    y = windowed_df[y_col].values
    rolling_slope = np.diff(y)
    rolling_slope_smooth = pd.Series(rolling_slope).rolling(
        window=3, center=True, min_periods=1
    ).mean().values

    # Find sign changes in slope (zero crossings)
    sign_changes = np.where(np.diff(np.sign(rolling_slope_smooth)))[0]
    for sc in sign_changes:
        sc_date = windowed_df.iloc[sc + 1]['window_start'] \
            if sc + 1 < len(windowed_df) else None
        transitions.append({
            'breakpoint_index': sc + 1,
            'date': sc_date,
            'detection_method': 'rolling_slope_sign_change',
            'direction': 'inflection',
        })

    return transitions
```

### Step 9: Classify Digital Maturity Stages

Map the trajectory and transitions to Digital Maturity stages.

```python
def classify_maturity_stages(windowed_df, transitions, y_col='activity_count',
                              features_df=None):
    """
    Classify the user's Digital Maturity stage at each time window.

    Stage classification uses BOTH activity volume AND trajectory shape.
    A user who posts frequently but shows no growth is a stable Participant,
    not a Leader.

    Thresholds below are STARTING POINTS. Calibrate to your data
    by examining the distribution of activity counts across your corpus.
    """
    df = windowed_df.copy()
    y = df[y_col].values

    # Compute rolling statistics for classification
    window_size = max(3, len(df) // 10)
    df['rolling_mean'] = pd.Series(y).rolling(
        window=window_size, min_periods=1, center=True
    ).mean()
    df['rolling_std'] = pd.Series(y).rolling(
        window=window_size, min_periods=1, center=True
    ).std().fillna(0)

    # Percentile-based thresholds (adapt to corpus)
    nonzero = y[y > 0]
    if len(nonzero) == 0:
        df['maturity_stage'] = 'Lurker'
        df['stage_confidence'] = 'high'
        return df

    p25 = np.percentile(nonzero, 25)
    p75 = np.percentile(nonzero, 75)

    def classify_window(row):
        rm = row['rolling_mean']
        if rm <= p25 * 0.5:
            return 'Lurker'
        elif rm <= p75:
            return 'Participant'
        else:
            return 'Community Leader/Moderator'

    df['maturity_stage'] = df.apply(classify_window, axis=1)

    # Confidence based on variance
    df['stage_confidence'] = df['rolling_std'].apply(
        lambda s: 'high' if s < p25 else ('medium' if s < p75 else 'low')
    )

    # Enhance with additional features if available
    if features_df is not None and 'unique_contexts' in features_df.columns:
        # Cross-community presence is a strong Leader signal
        for idx in df.index:
            if idx < len(features_df):
                contexts = features_df.iloc[idx].get('unique_contexts', 0)
                if contexts >= 5 and df.loc[idx, 'maturity_stage'] == 'Participant':
                    df.loc[idx, 'maturity_stage'] = 'Community Leader/Moderator'

    # Report stage distribution
    stage_counts = df['maturity_stage'].value_counts()
    print("\n=== Digital Maturity Stage Distribution ===")
    for stage, count in stage_counts.items():
        pct = 100 * count / len(df)
        print(f"  {stage}: {count} windows ({pct:.1f}%)")

    # Identify current stage (most recent stable window)
    recent = df.tail(max(3, window_size))
    current_stage = recent['maturity_stage'].mode().iloc[0] if len(recent) > 0 else 'Unknown'
    print(f"\nCurrent stage (recent {len(recent)} windows): {current_stage}")

    return df
```

### Step 10: Validate Transitions Against Activity Metadata

Do not rely solely on statistical change points. Cross-reference detected transitions with qualitative changes in the activity data.

```python
def validate_transitions(windowed_df, transitions, activity_df,
                          date_col='date'):
    """
    Check whether detected transitions correspond to observable
    changes in activity patterns, not just volume.

    Validation signals:
    - New content types appearing after transition
    - New communities/contexts entered
    - Change in content length or depth
    - External events (platform changes, life events) -- note but cannot detect programmatically
    """
    validated = []
    for tr in transitions:
        bp_date = tr.get('date')
        if bp_date is None:
            continue

        # Look at activity before vs after transition
        before = activity_df[
            pd.to_datetime(activity_df[date_col]) < bp_date
        ]
        after = activity_df[
            pd.to_datetime(activity_df[date_col]) >= bp_date
        ]

        validation = {'transition': tr, 'signals': []}

        # Check content type diversity change
        type_col = next(
            (c for c in ['type', 'content_type', 'action_type']
             if c in activity_df.columns), None
        )
        if type_col:
            types_before = set(before[type_col].dropna().unique())
            types_after = set(after[type_col].dropna().unique())
            new_types = types_after - types_before
            if new_types:
                validation['signals'].append(
                    f"New activity types after transition: {new_types}"
                )

        # Check context/community diversity change
        ctx_col = next(
            (c for c in ['subreddit', 'channel', 'forum', 'category']
             if c in activity_df.columns), None
        )
        if ctx_col:
            ctx_before = before[ctx_col].nunique() if len(before) > 0 else 0
            ctx_after = after[ctx_col].nunique() if len(after) > 0 else 0
            if ctx_after > ctx_before * 1.5:
                validation['signals'].append(
                    f"Context diversity increased: {ctx_before} -> {ctx_after}"
                )

        # Check content length change
        if 'body' in activity_df.columns:
            len_before = before['body'].dropna().str.len().mean()
            len_after = after['body'].dropna().str.len().mean()
            if not np.isnan(len_before) and not np.isnan(len_after):
                if len_after > len_before * 1.3:
                    validation['signals'].append(
                        f"Content length increased: {len_before:.0f} -> {len_after:.0f} chars"
                    )

        validation['validated'] = len(validation['signals']) > 0
        validated.append(validation)

    confirmed = sum(1 for v in validated if v['validated'])
    print(f"\nTransition validation: {confirmed}/{len(validated)} confirmed "
          f"by activity metadata changes")

    return validated
```

### Step 11: Write Report

Write all findings to `docs/analysis/11-longitudinal-growth-curves.md`. See Report Output Template below.

## Report Output Template

The final report MUST be written to `docs/analysis/11-longitudinal-growth-curves.md` with this structure:

```markdown
# Longitudinal Growth Curve Analysis

## Methodology
- Data source: [describe activity dataset, platform, date range]
- Timeline span: [start date] to [end date] ([N] months/years)
- Window aggregation: [weekly/biweekly/monthly], [N] total windows
- Windows with zero activity: [N] ([%])
- Models fitted: linear, logistic, piecewise
- Model selection criterion: BIC (lower = better)
- Date of analysis: [date]

## Raw Trajectory Summary
- Total activity items: [N]
- Activity rate: [mean] per window (std: [value])
- Trend (visual): [increasing / decreasing / plateau / mixed]
- [Path to trajectory plot if generated]

## Model Comparison

| Model | AIC | BIC | Delta BIC | R-squared | Rank (BIC) |
|-------|-----|-----|-----------|-----------|------------|
| [type] | [value] | [value] | [value] | [value] | [rank] |

**Selected model:** [type] (Delta BIC = [value] over second-best; [interpretation of evidence strength])

**Model selection rationale:** [Explain why selected model is preferred. If AIC and BIC disagree, note this and explain preference.]

### Selected Model Details
- Parameters: [list model parameters and values]
- Interpretation: [plain language explanation of trajectory shape]
- Goodness of fit: R-squared = [value]

### Alternative Model Notes
[Brief note on each non-selected model: why it was inferior or where it disagreed]

## Phase Transitions

### Detected Transitions
| # | Date | Window Index | Pre-Slope | Post-Slope | Direction | Validated? |
|---|------|-------------|-----------|------------|-----------|------------|
| 1 | [date] | [idx] | [slope] | [slope] | [accel/decel] | [yes/no] |

### Transition Validation
[For each transition, describe the supporting metadata evidence or lack thereof]

### Transitions NOT Detected
[Note if the model found no transitions -- this itself is a finding. Possible explanations:
 gradual change without abrupt shifts, insufficient data, or genuinely stable trajectory]

## Digital Maturity Classification

### Stage Distribution Over Time
| Stage | Windows | Percentage | Date Range |
|-------|---------|------------|------------|
| Lurker | [N] | [%] | [earliest] - [latest] |
| Participant | [N] | [%] | [earliest] - [latest] |
| Community Leader/Moderator | [N] | [%] | [earliest] - [latest] |

### Current Stage: [Stage Name]
- Based on most recent [N] windows
- Confidence: [high/medium/low]
- Supporting evidence: [activity level, diversity, trajectory direction]

### Stage Trajectory Narrative
[1-3 paragraphs describing the user's evolution: when they transitioned between stages,
 what evidence supports each transition, and what the overall trajectory shape reveals]

## Activity Feature Trends
[If multi-dimensional features were computed (content length, context diversity, etc.),
 show how they evolved alongside the primary activity count]

## Limitations and Caveats
- [Timeline too short for confident long-term modeling if applicable]
- [Large gaps in activity if applicable -- see gap analysis below]
- [Model fit quality -- low R-squared means trajectory is noisy]
- [Activity volume is a proxy, not a direct measure of maturity]
- [Platform changes or life events may explain transitions better than maturity evolution]
- [Thresholds for stage classification are corpus-relative, not absolute]
- [This analysis describes historical trajectory; it does NOT predict future behavior]

## Confounding Factors
[Note any known or suspected confounding factors:
 - Platform design changes that affected activity metrics
 - Account age effects (new-user enthusiasm vs. veteran fatigue)
 - Seasonal patterns in platform usage
 - Data export limitations (incomplete history)]

## Downstream Use
[How these findings feed into persona synthesis, weighted engagement scoring,
 or archetype assignment. Reference other analysis skills as appropriate.]
```

## Good Patterns

- **Fit multiple model types and compare:** Never accept the first model that "looks right." Always fit linear, logistic, and piecewise, then let AIC/BIC decide.
- **Use AIC/BIC for model selection:** Information criteria balance fit quality against complexity. They prevent overfitting to noise.
- **Identify change points explicitly:** Stage transitions are the primary deliverable. A model with good overall fit but no transition detection misses the point.
- **Windowed aggregation before fitting:** Raw timestamps are too noisy. Aggregate into regular windows (weekly/monthly) with zero-activity windows preserved.
- **Validate transitions against activity metadata:** A statistical change point is a hypothesis. Confirm it by checking whether content types, contexts, or content depth also changed.
- **Document model selection rationale:** Future readers (including future Claude instances) need to know WHY a model was chosen, not just which one.
- **Use percentile-based thresholds:** Absolute activity thresholds do not generalize across platforms. Calibrate stage boundaries to the user's own distribution.

## Anti-Patterns

| Anti-Pattern | Why It Fails | Instead |
|--------------|-------------|---------|
| Fitting a single model without alternatives | You cannot know if linear fits well without comparing to logistic and piecewise | Always fit all three; compare with AIC/BIC |
| Overfitting to noise with high-degree polynomials | Degree-3+ polynomials fit training data well but produce absurd extrapolations and unstable segment interpretations | Cap at degree 2 (quadratic) for polynomials; prefer piecewise for complex shapes |
| Treating activity volume as sole maturity indicator | High-volume posting without depth, diversity, or reciprocity is not maturity -- it may be spam or obsessive single-topic engagement | Use multi-dimensional features: activity count + content length + context diversity + action type diversity |
| Ignoring gaps in activity | Dropping zero-activity windows biases trajectories upward and hides lurking periods | Fill time series with zero-activity windows using `reindex` with full date range |
| Assuming linear growth | Most users do not grow linearly; S-curves and piecewise transitions are far more common in community participation research | Let the data decide -- fit multiple models and compare |
| Predicting future behavior from the trajectory | Growth curves model historical patterns; extrapolation is unreliable, especially beyond the observed range | Explicitly state the analysis is retrospective. Do NOT forecast. |
| Claiming psychological maturity from activity patterns | "They post more, therefore they are more mature" conflates observable behavior with internal state | Use the term "Digital Maturity stage" to specifically denote observable behavioral patterns, not psychological claims |
| Using raw AIC/BIC values without delta comparison | Individual AIC/BIC values are meaningless; only the difference between models matters | Always compute and report Delta BIC from the best model |

## Boundaries

**This skill SHOULD:**
- Model activity trajectories using multiple candidate growth curve models
- Identify phase transitions via change point detection and slope analysis
- Classify current Digital Maturity stage (Lurker / Participant / Community Leader/Moderator)
- Compare models using AIC/BIC and document selection rationale
- Validate detected transitions against qualitative activity metadata changes
- Fill zero-activity gaps in time series
- Produce a structured report at `docs/analysis/11-longitudinal-growth-curves.md`

**This skill should NOT:**
- Predict future behavior or extrapolate trajectories beyond observed data
- Claim psychological maturity from activity volume patterns
- Ignore confounding factors (platform design changes, account age effects, seasonal patterns, life events)
- Use absolute activity thresholds that do not generalize across platforms
- Skip model comparison and accept a single model without alternatives
- Treat change points as definitive stage boundaries without metadata validation
- Make causal claims about what drove transitions ("they became a leader because...")

## Insufficient Data Handling

```dot
digraph insufficient_data {

…(truncated)