As noted in the Mott MacDonald study:
'optimism bias' can be defined as the 'tendency for a project's costs and duration to be underestimated and/or benefits to be overestimated.'5
Prior to discussing specific results of the present study it is worthwhile reflecting on the interpretation of normalised results and the importance of bias. The normalising of results and the investigation of a robust sample should result in a spread of results that takes the form of a normal distribution. If results are deemed to be without bias, the variance of a particular sample may be statistically acceptable. If, on the other hand, samples have a bias this is an indication of inaccurate initial assumptions and inadequate processes. A bias would suggest that the true average of samples is not zero but rather a different outcome that corresponds to a different distribution.
___________________________________________________________________
5 Mott McDonald (July, 2002), Review of Large Public Procurement in the UK, p.S-1.