Can Software Schedules Be Estimated?

J.P.Lewis writes " Is programming like manufacturing, or like physics? We sometimes hear of enormous software projects that are canceled after running years behind schedule. On the other hand, there are software engineering methodologies (inspired by similar methodologies in manufacturing) that claim (or hint at) objective estimation of project complexity and development schedules. With objective schedule estimates, projects should never run late. Are these failed software projects not using proper software engineering, or is there a deeper problem?" Read on for one man's well-argued answer, which casts doubt on most software-delivery predictions, and hits on a few of the famous latecomers.

"A recent academic paper Large Limits to Software Estimation (ACM Software Engineering Notes, 26, no.4 2001) shows how software estimation can be interpreted in algorithmic (Kolmogorov) complexity terms. An algorithmic complexity variant of mathematical (Godel) incompleteness can then easily be interpreted as showing that all claims of purely objective estimation of project complexity, development time, and programmer productivity are incorrect. Software development is like physics: there is no objective way to know how long a program will take to develop."

Lewis also provides a link to this "introduction to incompleteness (a fun subject in itself) and other background material for the paper."

Comments

Re:from a Consulting viewpoint..

[markmoss]

To accuratly plan a software release you must have the project, and all it's complexities and nuances down COLD. otherwise you are not giving an estimation, you are giving a guess based upon incomplete knowledge.

The bulk of the work of programming consists of getting all the complexities and nuances down cold. Once you really and completely understand what is required, coding is trivial.

This leads to a thoroughly unrealistic method of estimating software costs:
1) Work for months on the specs.
2) Get the customer to sign on to those incredibly detailed specs, even though he doesn't understand them.
3) Go and code it, no spec changes allowed.

8-)

The article mainly talks about the mathematics of estimating complexity. This is a lot like the proof that you cannot determine when or whether a computer program will end -- it's true for pathological programs, but it has little relevance for the real world. You try to write the code so the conditions for the program to end are clear. If it gets into an endless loop, you probably got a conditional expression backwards and you'll recognize it immediately once you figure out which loop is running endlessly... Likewise, there may be well-defined specifications for which it is impossible to estimate the coding time, but the usual problem is poorly-defined specs, which obviously makes any estimate a guess.