Planning Fallacy

The underestimation of the time needed to complete a project or programme of works. There are many variables which can impact on a planned programme of works: especially by being too optimistic on the timings; the impact of external factors – for example the weather, supply issues, reliance on external contractors; less efficient machinery or machinery breakdowns and malfunctions; changing staff motivations or availability due to illness, holidays, new training requirements, leaving employment, skills of newly recruited staff, abilities of agency staff, tiredness, being over-worked; predictions made from previous activities, but which had significant hidden differences in the detail of the programme; as well as not fully understanding what is required to achieve the undertake and complete the required project.
(See also Roger Buehler, Dale Griffin, and Michael Ross, (1994) , ‘Exploring the "Planning Fallacy": Why People Underestimate Their Task Completion Times’, Journal of Personality and Social Psychology 1994, Vol. 67, No. 3.366-381, https://web.mit.edu/curhan/www/docs/Articles/biases/67_J_Personality_and_Social_Psychology_366,_1994.pdf, accessed 2nd December 2023)