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My list of issues with estimation of software effort, costs and schedules.
See McConnell for another list (better).
Cost models aren’t perfectThe official “engineering” way of estimating proposed by CMM and others work in three steps:
Kemerer has shown that these models have a huge error factor (up to 500%) even if you know the size (step 1 is perfect).
Guessing program size
There is no objective way to estimate the size of a program given the requirements
This article by Lewis provides the reasoning behind this statement. Even using something like Parnas’ tables to specify all inputs and outputs of the program, the range between minimum size and the upper bound is huge.
Productivity is dificult to quantify
Even if you measure size and development time after the project is finished you still don’t know the productivity, as the program might be unnecessarily big (i.e. could have been done in 200 lines instead of 5000).
Personal factors have a big effect
COCOMO advertises an effort/size estimate error of 50% after requirements and 10% after detailed design. Even so, as McConnell points out:
The seven personnel-oriented factors collectively … exert a staggering influence range of 25.8!
Better people! Try it out here
See this honest article at the 7th International Forum on COCOMO and Software Cost Modeling to see the problems of applying something like COCOMO in your organisation.
So, what can we and what should we do?