Problems with over and underestimates

Parkinson's law was originally expounded in C. Northcote Parkinson's tongue-in-cheek book Parkinson's Law, John Murray, 1957.

A project leader such as Amanda will need to be aware that the estimate itself, if known to the development team, will influence the time required to implement the system. An over-estimate might cause the project to take longer than it would otherwise. This can be explained by the application of two 'laws'.

Parkinson's Law 'Work expands to fill the time available', which implies that given an easy target staff will work less hard.

Brooks' Law The effort required to implement a project will go up disproportionately with the number of staff assigned to the project. As the project team grows in size so will the effort that has to go into management, co-ordination and communication. This has given rise, in extreme cases, to the notion of Brooks'

Law: 'putting more people on a late job makes it later'. If there is an over-estimate of the effort required then this might lead to more staff being allocated than are needed and managerial overheads will be increased. This is more likely to be of significance with large projects.

Some have suggested that while the under-estimated project might not be completed on time or to cost, it might still be implemented in a shorter time than a project with a more generous estimate. There must, however, be limits to this phenomenon where all the slack in the project is taken up.

The danger with the under-estimate is the effect on quality. Staff, particularly those with less experience, might respond to pressing deadlines by producing work which is sub-standard. Since we are into laws, this might be seen as a manifestation of Weinberg's zeroth law of reliability: 'if a system does not have to

Brooks' law comes from The Mythical Man-month that has been referred to already.

See T. K. Hamid and S. E. Madnick 'Lessons learnt from modeling the dynamics of software development' in C. F. Kemerer (ed.) Software Project Management, Irwin, 1997.

Barry Boehm devised the COCOMO estimating models, which are described later in this chapter.

be reliable, it can meet any other objective'. In other words, if there is no need for the program actually to work, you can meet any programming deadline that might be set! Sub-standard work might only become visible at the later, testing, phases of a project, which are particularly difficult to control and where extensive rework can have catastrophic consequences for the project completion date.

Because of the possible effects on the behaviour of development staff caused by the size of estimates, they might be artificially reduced by their managers to increase pressure on staff. This will work only where staff are unaware that this has been done. Research has found that motivation and morale are enhanced where targets are achievable. If, over a period of time, staff become aware that the targets set are unattainable and that projects are routinely not meeting their published targets, then this will help to destroy motivation. Furthermore, people like to think of themselves as winners and there is a general tendency to put success down to our own efforts, while failure is blamed on the organization.

In the end, an estimate is not really a prediction, it is a management goal. Barry Boehm has suggested that if a software development cost is within 20% of the estimated cost estimate for the job then a good manager can turn it into a self-fulfilling prophecy. A project leader like Amanda will work hard to make the actual performance conform to the estimate.

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  • johanna
    What are the problems with over and under estimate?
    3 years ago
  • Steffen
    How over and underestimation?
    1 year ago
  • frans
    What are the problen with over and under estimates?
    1 year ago

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