Sherman Kent

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1. 01

   A probability word with no number behind it is read differently by every reader; the remedy is an agreed scale with numerical ranges.

   Words of Estimative Probability · 1964

   A probability word with no number behind it is a projective test, not an estimate.

   In March 1951 National Intelligence Estimate 29-51 called an attack on Yugoslavia that year a serious possibility. Kent, a member of the Board of National Estimates that had approved the text, had odds of about 65 to 35 in favour of an attack in mind; a State Department reader had understood a much lower chance, and Kent's colleagues each held a different figure, from 20 to 80 up to 80 to 20. His answer was a table: a few expressions, each tied to a range — probable around 75 per cent, almost certain around 93 per cent. Today ICD 203 requires a seven-step scale, from almost no chance (1–5 per cent) to almost certain (95–99 per cent). The British PHIA yardstick also has seven steps, from remote chance (up to about 5 per cent) to almost certain (from about 95 per cent), but deliberately leaves gaps between them. In practice: pick the number first, then the word from the scale; do not mix sets of terms; if you cannot defend the word, you do not yet have a judgement. The trap is what Kent called the poets' camp: phrases such as may well or distinctly possible, which suggest odds without committing to them.

   “It was another jolt to find that each Board member had had somewhat different odds in mind and the low man was thinking of about 20 to 80, the high of 80 to 20.”

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