What percentage of the total cost of ownership of an IT system falls on O & M?

Question

I recall that in the past a lot of research and statistics said that "90% of the cost of an IT system falls on the O&M phases" (compared to the development stages), etc., but where are the sources for this?

Does anyone have white paper or research paper links to support this? I know that this is generally true, but I want a source and a number (%) based on reality, not memory / guessing.

Answer 1

Fred Brooks in the "Mythical Human Month" gives the figure "40% or more" for the service, I did not find an explicit source for this figure.