Lehman ‘laws’ of software evolution

The so called Lehman laws of software evolution originated in a 1968 study, and evolved during the 1970s; the book “Program Evolution: processes of software change” by Lehman and Belady was published in 1985.

The original work was based on measurements of OS/360, IBM’s flagship operating system for the computer industries flagship computer. IBM dominated the computer industry from the 1950s, through to the early 1980s; OS/360 was the Microsoft Windows, Android, and iOS of its day (in fact, it had more developer mind share than any of these operating systems).

In its day, the Lehman dataset not only bathed in reflected OS/360 developer mind-share, it was the only public dataset of its kind. But today, this dataset wouldn’t get a second look. Why? Because it contains just 19 measurement points, specifying: release date, number of modules, fraction of modules changed since the last release, number of statements, and number of components (I’m guessing these are high level programs or interfaces). Some of the OS/360 data is plotted in graphs appearing in early papers, and can be extracted; some of the graphs contain 18, rather than 19, points, and some of the values are not consistent between plots (extracted data); in later papers Lehman does point out that no statistical analysis of the data appears in his work (the purpose of the plots appears to be decorative, some papers don’t contain them).

One of Lehman’s early papers says that “… conclusions are based, comes from systems ranging in age from 3 to 10 years and having been made available to users in from ten to over fifty releases.“, but no other details are given. A 1997 paper lists module sizes for 21 releases of a financial transaction system.

Lehman’s ‘laws’ started out as a handful of observations about one very large software development project. Over time ‘laws’ have been added, deleted and modified; the Wikipedia page lists the ‘laws’ from the 1997 paper, Lehman retired from research in 2002.

The Lehman ‘laws’ of software evolution are still widely cited by academic researchers, almost 50-years later. Why is this? The two main reasons are: the ‘laws’ are sufficiently vague that it’s difficult to prove them wrong, and Lehman made a large investment in marketing these ‘laws (e.g., publishing lots of papers discussing these ‘laws’, and supervising PhD students who researched software evolution).

The Lehman ‘laws’ are not useful, in the sense that they cannot be used to make predictions; they apply to large systems that grow steadily (i.e., the kind of systems originally studied), and so don’t apply to some systems, that are completely rewritten. These ‘laws’ are really an indication that software engineering research has been in a state of limbo for many decades.