Part of the (incorrect) folklore of software engineering is that more money is spent on maintaining an application than was spent on the original development.

Bossavit’s The Leprechauns of Software Engineering does an excellent job of showing that the probably source of this folklore did not base their analysis on any cost data (I’m not going to add to an already unwarranted number of citations by listing the source).

I have some data, actually two data sets, each measuring a different part of the problem, i.e., 1) system lifetime and 2) maintenance/development costs. Both sets of measurements apply to IBM mainframe software, so a degree of relatedness can be claimed.

Analyzing this data suggests that the average maintenance/development cost ratio, for a IBM applications, is around 0.81 (code+data). The data also provides a possible explanation for the existing folklore in terms of survivorship bias, i.e., most applications do not survive very long (and have a maintenance/development cost ratio much less than one), while a few survive a long time (and have a maintenance/development cost ratio much greater than one).

At any moment around 79% of applications currently being maintained will have a maintenance/development cost ratio greater than one, 68% a ratio greater than two and 51% a ratio greater than five.

Another possible cause of incorrect analysis is the fact we are dealing with ratios; the harmonic mean has to be used, not the arithmetic mean.

Existing industry practice of not investing in creating maintainable software probably has a better cost/benefit than the alternative because most software is not maintained for very long.