Words of estimative probability

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Words we use to communicate uncertainty.

Sherman Kent

Sherman Kent was a CIA analyst in the Cold War and promoted a scheme of probabilities for specific words and phrases[1]. This is the scheme used in Agile's Risk* mobile app.

100% Certain
93% give or take about 6% Almost certain
75% give or take about 12% Probable
50% give or take about 10% Chances about even
30% give or take about 10% Probably not
7% give or take about 5% Almost certainly not
0% Impossible

Pete Rose

Figure 3 from McLane et al 2008[2]

This is Figure 18 in Rose (2001[3]).

100% Will occur
90–97% Virtual certainty
60–79% Reasonable geologic confidence
40–59% Significant uncertainty (50% = "toss up")
20–39% Less likely present than absent
1–19% High-risk geologic factor
0% Won't occur

Rose's consulting firm performed a study similar to the O'Brien paper mentioned below. They reported on it in McLane et al (2008)[2]. Here (right) they report on the range of confidence levels associated with words and phrases:


The Intergovernmental Panel on Climate Change has recommended[4] some language for communicating likelihood.

>99% Virtually certain
>90% Very likely
>66% Likely
33 to 66% About as likely as not
<33% Unlikely
<10% Very unlikely
<1% Exceptionally unlikely


O'Brien (1989[5]) reported on the inherent ambiguity of WEPs in general medical practice. He interviewed 56 doctors about twenty-three words and phrases, asking them to place them on a probability scale. He then used the interquartile range of the responses as an indication of ambiguity. O'Brien also compared the interquartile range to a 3-point rating of ambiguity, as given by the respondents. Here's how his data plot:

WEP Ambiguity vs prob.png WEP IQrange vs prob.png

External links


  1. Sherman Kent, Words of Estimative Probability, CIA Studies in Intelligence, Fall 1964. As a web page.
  2. 2.0 2.1 McLane, Mark, James Gouveia, Gary P. Citron, James MacKay, and Peter Rose (2008), Responsible Reporting of Uncertain Petroleum Reserves. AAPG Bulletin 92 (10), p 1431–1452. Also available online at the SEC.
  3. Rose, P (2001), Risk Analysis and Management of Petroleum Exploration Ventures, AAPG Methods in Exploration Series, number 12.
  4. IPCC (2005), Guidance Notes for Lead Authors of the IPCC Fourth Assessment Report on Addressing Uncertainties, July 2005. PDF version.
  5. O'Brien, B (1989), Words or numbers? The evaluation of probability expressions in general practice. Journal of the Royal College of General Practitioners 39, p 98–100, March 1989. Link to PDF.