Developers have to estimate the amount of time required to complete their features. This is at the same time a simple and a frightening task because the business depends on them. There is no ultimate way to make correct estimation every time but it is possible to aim for a good evaluation.

But first, what is an estimate in the software development environment ?

In some cases it is a commitment, you must achieve it ! You have to be absolutely certain (100%) to commit yourself into a deadline. Other people will make plan based on your commitments so you definitely don’t want to miss them. Otherwise you will look as a dishonest person and you are doing harm (see Professionalism chapter).

In other cases an estimate is… an estimate ! It is a guess, you do not promise anything and you do not commit yourself. If you already had to quantify the time a development task will take you certainly know how difficult it is. Most of the time there are too many variables in play to make a proper estimation. And most of all an estimate is not a number, it is a probability distribution ! The concept is explains with the following figure (fig. 1).

In this example the task has 50% chance to take 3 days but it can also take 4-5 days or even 10 days (the Murphy’s law is never far away). With a distribution the business has data to rely on for their work and the development team does not have an inconsistent deadline. It is also important not to commit yourself without willing to do, especially if you use the verb “try” (see chapter 2).

Program Evaluation and Review Technique (PERT) offers an effective way to convert estimates into probability distributions suitable for the business team and for the managers. When estimating a task 3 numbers must be provided, it is a trivariate analysis :

O : Optimistic Estimate. Everything goes as planned, nothing as come into the way. In the chart example, O = 2.

N : Nominal Estimate. The value with the highest probability, in our example, N = 3.

P : Pessimistic Estimate. Everything goes wrong, very low percentage of chance to append. P = 10.

With these 3 values it’s possible to describe the probability distribution of the task with the following formula :

µ = (O + 4N + P) / 6

µ (mu) gives us the expected duration of the task. For the example this value is equal to 4 days. But this is just an average based on some coefficients and cannot be used as a deadline, it is a hint. Moreover the pessimistic estimate high value increase this average.

σ = (P – O) / 6

In this second formula, σ (sigma) represents the standard deviation of the task. It is a measure to specify how uncertain the task is, it is equal to 1.33 days with our sample data. The higher this value is the higher the risk is.

With the PERT technique the estimate for the task is the combination of the two values : 4 | 1.33. You cannot take one without the other.

Agile methodologies have given us several techniques to estimate tasks within a small development team. You can try a method called Wideband Delphi for example or the Planning poker approach or even the Affinity Estimating technique. All of them are based on agreement to provide meaningful estimates and you can combine them with PERT. To ease the process of estimating it is advisable to transform larger task into several atomic tasks, this way the team can be more accurate.

Professionals developers provide practical estimates, do not make promise they can’t keep and commitments that can’t be meet. An estimate is a probability and not a deadline !

See you next time for “Pressure“ !