Why and How to State the Credibility of Estimations

Type: Principles, Rules
Sources: T. Gilb's work on Impact Estimation Tables, see http://www.gilb.com/; my own experience


Have you seen presentations suggesting a project or other endeavour with the purpose of convincing someone that the project would be the right thing to do? More often than not these presentations include shiny numbers of relatively low cost and relatively high benefit (of course). But how often does the presenter tell something about the Credibility of the numbers?
I believe every decision maker needs some assessment of credibility. If you don't provide him with how believable your evidence is, the decision maker might turn to other sources, like "what's this guy's success record?", "I don't believe her so I will resort to a very tight budget in order not to risk too much", etc.

Same problem with any estimate you prepare and present. These Principles explain why you should always give credibility information, and the Rules explain how you can do it.

" An estimate that is 100% certain is a commitment." — Unknown

Credibility information DEFINED AS any evidence or number that expresses how certain we believe a statement is

Principles and Notes

P1: Estimates are assumptions about the future, therefore they must be uncertain.

"Unfortunately, tomorrow never gets here." - Leo Babauta

P2: Predictions are a difficult matter.

The farther away the state we predict, the more difficult, the riskier.

P3: Credibility information is a measure of how certain we are about a predicted state.
P4: Decision makers need some assessment of the risk involved in the decision.

What can happen in the worst case? (It is equally valid to ask for the best case, but seriously, the best case happens far too seldom to take it into account)

"Most people really like to be able to sleep at night." — Heuser's Rule

P5: The clearer you state uncertainty,
  • the easier it is to distinguish between different strategies
  • the more likely other people believe you
  • the clearer you yourself see the risk involved
  • the better you can argue about a strategy
  • the easier it is to learn something about the risk involved and to do something about it
  • the clearer you give or take responsibility for an endeavour

Note: While the decision maker obviously accepts a risk involved by seeing uncertainty numbers, there are decision makers who don't like the idea at all. I guess this is a reason why credibility information isn't requested very often.

P6: If credibility information is missing,

we by default assume the number or the source is not credible at all.
Note: Maybe the presenter just forgot to provide the information, and then it shouldn't be a problem. We can send him to get it.


R1: Any number that expresses some future state should be accompanied by credibility information.
R2: If you predict a value on a scale, like "end date", "targeted budget", "performance improvement", or "maintenance cost", give a range of that number.


  • It's useful to do this in a plus/minus X% fashion. The percentage is a clear and comparable signal to the audience.
  • It is not mandatory to give the best/worst case estimates an equal range. How about "we are sure to complete this in 10 weeks, minus 1 plus 2"?
  • In Rapid Development, Steve McConnell actually suggests that you communicate estimates in a range that gets smaller over time, with the larger number first. "Six to four months" sounds strange - but if you say the smaller number first, people tend to forget the larger one.)
R3: If you provide evidence, say where you have it from, or who said so, and what facts (numbers, history, written documentation) lead to this conclusion.

Note: Do this in the backup slides for example. At least you should be able to pull it out of the drawer if requested.

R4: Consider using a safety margin, like factor 2 (bridge builders), factor 4 (space craft engineers).


  • The margin is an expression of how much risk your are willing to take. Thus, it is a way of controlling risk.
  • Use margins whenever you find a number or source is not very credible (like < 0.4), you don't have any historic data, or if there's no economic way of achieving higher credibility.
  • Safety Margins do not necessarily increase real costs (but planned costs)
R5: Always adjust your cost/benefit-ratios by a credibility factor.

A simple scale would be {0.0 - guess, no real facts available, 0.5 - some facts or past experience available, 1.0 - scientific proof available}
A more sophisticated scale would be
{0.0 Wild guess, no credibility
0.1 We know it has been done somewhere (outside the company)
0.2 We have one measurement somewhere (from outside the company)
0.3 There are several measurements in the estimated range (outside the company)
0.4 The measurements are relevant to our case because <fact1, fact2> with credibility <x, y> (don't get trapped in recursion here)
0.5 The method of measurement is considered reliable by <whom>
0.6 We have used the method in-house
0.7 We have reliable measurements in-house
0.8 Reliable in-house measurements correlate to independent external measurements
0.9 We have used the idea on this project and measured it
1.0 Perfect credibility, we have rock solid, contract-guaranteed, long-term, credible experience with this idea on this project and, the results are unlikely to disappear}



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