Friday, March 20, 2009

Software Estimation v.1

I recommend to everyone who is involved in any way with software development to read the book "Software Estimation: Demystifying the Black Art" by Steve McConnell. I laughed, I cried, it changed my life. Well maybe not the first two, but it did change the professional side of my life.

There's so much to software estimation that one article cannot do it justice; especially if the objective of the article is to keep it short and concise. This first of probably many blog entries on software estimation will be dedicated to distinguishing between an estimate and a commitment. This is a vital point because the two are often used interchangeably.

Estimating is an unbiased analytical process. With regards to software development, the objective is to approximate the amount of time and/or cost to develop software to solve a problem or satisfy a specific need. As an approximation, an estimate should be communicated as a range, e.g. "it is likely that this project will be delivered in 3-5 weeks".

A commitment is much more definitive. It is an agreement to deliver specific functionality on a specific date for a specific cost, e.g. "the project will be delivered in 6 weeks."

Related to a commitment is a target. A target is a biased process based on the goals of the business, e.g. "We need this software delivered by June 15 to demo at the premier industry convention!".

In my experience, more often than not estimates are expressed as commitments which are influenced by the business driven target.

Communicating estimates as a single point number vs as a range is misleading (in most cases unintentionally). Every software development effort is, in actuality, an invention and an invention cannot be guaranteed to be completed on a certain day for a certain cost. As we all know, our estimates are not 100% accurate. However, if we communicate estimates as a single point number we are implying 100% accuracy.

Developing high/low cost estimates is fairly common and as you can probably guess I recommend communicating both to your customers. There are many techniques to quantify, thus making credible, high/low estimates. Although estimation by intuition is one estimation technique - and is probably the most frequently used - it is the least reliable. I'll be writing about some of many techniques I garnered from Steve McConnell's book in future blog entries.

On the other hand, dates are rarely communicated as high/low estimates. Typically they are communicated as commitments (single point date) which often result in very long days for developers, negotiating with the client to deliver at a later date, removing features from scope, providing a poor quality product, or a combination of some or all of those. I have a question for you, when was the last time you presented your delivery dates as probabilities, e.g. "there is a 25% probability of us delivering the software in 11 weeks and a 98% probability that we'll deliver it in 15 weeks."? The techniques for quantifying those probability statements will come in later articles too.

I'm interested in hearing about your issues/resolutions to software estimation problems.


  1. That's a great book. I love books that pack in lots of info without lots of fluff.

  2. It's on my desk at all times. I must refer to it at least once a month. Thanks for the feedback!


Web Analytics