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