Wednesday, August 13, 2014

Software Development Estimates

Programmer estimates are notoriously bad. Having been in the position to give estimates and to be blocked by another's faulty estimate, the whole process is deeply frustrating for everyone.

I've been trying to figure out what is going on with task estimation for a while. There are plenty of theories on the internet. Many say programmers are just too optimistic. That's undoubtedly part of it but in terms of a theory with predictive and explanatory power it's like saying that programmers underestimate tasks because their estimates are always too short.

A group of psychologists first proposed a theoretical basis for.. I feel I should say "optimism".. in a 1979 paper called "Intuitive prediction: biases and corrective procedures". This pattern of optimist task estimation has shown up in tax form completion, school work, origami and a bunch of other things. From this we can conclude that programmers don't suck at task estimation, human beings suck at task estimation. A condemnation of humanity but an encouraging result for those who still cling to the ridiculous notion that programmers are people. The phenomenon is called the "Planning Fallacy" and Wikipedia has a great summary if you're interested. I estimate it will take you 5 seconds to read so go ahead.



Optimistic estimates are bad enough, but organizations will often add estimate distortions of their own.

  1. Fabricate a schedule without consulting anyone who will be taking part of the project.
  2. Ask developers for estimates and then cut them down because they are too high or those wacky developers are "padding" their estimates.
  3. Ask for estimates from developers without there being a detailed plan to estimate.
  4. Use estimates for a plan that's not being used anymore. Sometimes this happens because of additional features were added. Sometimes it's because the team you are integrating with hit a technical wall and had to change everything. Sometimes people are still using the pre-project estimates when the task estimates are available.
  5. Ask a lead developer for an estimate then give the job to an intern.

I've been on more projects than I can count and it didn't seem to matter who was on the project or how many times they've been through the process because at least one of these things happened. The last project I was on had the last three estimation pathologies show up at one point or another.

Once a project is behind schedule and you have a deadline to meet, all the options start to suck.


There have been many attempts to fix this estimate mess some more successful than others. The current best practice is to use some variant of agile/scrum. I say "some variant" because agile programming has many forms and not all of them are understood properly.

Agile software development turns the estimate problem on its head by admitting that estimates are likely to be wrong and that features will be added or removed so why not deal with it. The first thing agile does is to try and compute a fudge factor for the task estimates. The assumption is that the estimates are relatively accurate they are just off by some constant factor X. If you can figure out this factor you can multiply all estimates by X and get a more accurate number. In practice this helps but ins't a panacea.

The second thing Agile does is it seeks to minimize the problems bad estimates cause. This may seem like giving up because it is. As far as I can tell, no one in the software development industry has managed to solve this problem outside of some very special cases. The best strategy is to plan for bad estimates.

With agile, programming task estimates are re-calculated on a weekly basis so that estimates can include the latest information as its discovered. The focus is on keeping estimates up to date and communicating them instead of realizing a feature is too large when it's impossible to do anything about it. Additionally, features are done in order of importance; there's an ordered product backlog with the most critical things at the top of the pile. This way, developers know what is important and what isn't and can work with that in mind. When the deadline comes around you're guaranteed to have the important things done instead of whatever was fun to program.

It's way too hard to give a good summary of Agile here so I'm going to point to some resources:

  • Wikipedia's Agile Software Development page is a good starting point.
  • You can also look at Scrum. Most industry standard agile best practices are some variant of Scrum.
  • Joel Spolsky advocates his version which he calls Evidence Based Scheduling. I wouldn't normally include this but the page has a good explanation of where typical task estimation goes wrong.
  • There are quite a few consulting groups that can help too.


Developers need a feedback loop for their estimates. At some point after the feature has been implemented (this includes debugging!), developers should get feedback as to the original estimate and the actual time taken. Most agile tools will try to capture that but it might miss important aspects like design time or the time it took to gather requirements. In any case, this information should be explicitly presented to everyone on the team (including managers). Developers rarely consider how long - in real time - things are taking or how it relates to their estimates. Compiling and presenting the numbers to the team at the end of a project, when they are likely to be receptive, is enough to start this thinking process. It also communicates that the organization cares about the accuracy of estimates. If you're curious about why something took so long, this is a good place to have that discussion.

Out of the many projects I have been on, none of them have outright failed despite the usual optimistic estimates and estimation pathologies. This is because I have always worked within organizations that that were flexible enough to work with bad estimates. I am completely convinced that protecting your project against bad estimates is a realistic approach to managing estimate risk. That said, better estimates are always welcome so watch out for organizational estimation pathologies and make sure the developers realize how long their tasks are actually taking vs their estimates.


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