Thorpe's Second Rule of Estimating

IT IS EITHER 11.6 TIMES BIGGER OR 1/11.6 TIMES SMALLER THAN THE USER THINKS.

Unfortunately, they underestimate 11.6 times more than they overestimate, but nonetheless for 8.67% of the time you can impress users. By the way the 11.6 is an average, the more senior the user the more he will underestimate the task to the point where I had a Chief Executive who was convinced that anything could be achieved with 'only one line of code'. Another strange aspect of the users perception is that it cannot be improved by any amount of training.