What percentage of the total cost of ownership for an IT system is incurred during O&M?

Question

I recall hearing lots of studies and statistics in the past that say something like "90% of the cost of an IT system is during O&M phases" (vs development phases) and so on, but where/what are the sources for this?

Does anyone have any white paper or research paper links to back this up? I know it's generally true but I want a source and a number (%) based on reality rather than memory/guess.

Answer 1

Fred Brooks in The Mythical Man Month gives a figure of "40% or more" for maintenance, I didn't find any explicit source for that figure.