Liberty91LTD Likelihood Language — Probability Yardstick
Every forward-looking assessment (prediction, forecast, risk statement) MUST use standardised likelihood language. Never use vague terms like "could", "might", or "may" without mapping them to a probability band.
The Yardstick
| Term | Probability Range | Usage |
|---|---|---|
| Almost certain | >95% | Use sparingly. Reserved for near-inevitable outcomes with overwhelming evidence. |
| Highly likely | 75-95% | Strong evidence supporting the assessment. Limited plausible alternatives. |
| Likely | 50-75% | The weight of evidence supports this outcome, but meaningful alternatives exist. |
| Realistic possibility | 25-50% | Plausible outcome that cannot be discounted. Evidence is mixed or limited. |
| Unlikely | 15-25% | Possible but not supported by the weight of evidence. |
| Highly unlikely | 5-15% | Very limited evidence suggests this outcome. Most evidence points elsewhere. |
| Remote | <5% | Theoretically possible but requires multiple unlikely conditions to align. |
How to Use
Correct usage
"It is highly likely (75-95%) that the threat actor will shift targeting to the financial sector in Q3, based on observed reconnaissance activity and historical pattern of seasonal targeting shifts."
"There is a realistic possibility (25-50%) that the exploit will be weaponised within 30 days, given the availability of a proof-of-concept but no evidence of active exploitation."
Incorrect usage
"The threat actor might target financial services." ❌ (No probability band) "It could happen in Q3." ❌ (Vague, unquantified) "We think it's likely but also possible that..." ❌ (Contradictory — pick one band)
Combining with Confidence
Likelihood and confidence are independent dimensions:
"We assess with moderate confidence that it is likely (50-75%) that Group X will conduct destructive operations against energy sector targets in the next 6 months."
- Moderate confidence: We have some evidence but key assumptions are not fully validated
- Likely: Based on available evidence, the probability falls in the 50-75% range
A low-confidence assessment CAN have a high likelihood term — it means "if our assessment is correct, the probability is high, but we're not fully sure our assessment is correct."
When NOT to Use Likelihood Language
- Statements of fact: "APT28 used Fancy Bear malware in this campaign" — this is a factual claim, not a prediction. Use confidence levels instead.
- Historical analysis: "The campaign began in January 2025" — past events don't need likelihood language.
- Definitions and descriptions: "SIGMA rules are detection rules written in YAML" — informational, not predictive.
Mapping Informal Language
If you catch yourself writing informal probabilistic language, map it:
| Informal | Map to |
|---|---|
| "could", "may", "might" | Determine which band and use proper term |
| "probably" | Likely (50-75%) or Highly likely (75-95%) |
| "possibly" | Realistic possibility (25-50%) |
| "unlikely" | Already a valid term — check it maps to 15-25% |
| "almost certainly" | Almost certain (>95%) |
| "there's a chance" | Determine probability and use proper term |
| "can't rule out" | Realistic possibility (25-50%) or Unlikely (15-25%) |
Common Mistakes
- Using multiple likelihood terms in one assessment ("likely or highly likely") — commit to one
- Hedging with "possible" instead of making a judgment — if evidence is weak, say "unlikely" with rationale
- Forgetting to include the percentage range in parentheses — always include for clarity
- Confusing likelihood with confidence (see confidence-levels skill)
- Using "almost certain" casually — this is >95% and should be rare