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Guesstimation

2008-09-24 08:43:54

At the initial planning stage the main objective is to get a realistic estimate of the time involved in the project. You must establish this not only to assist higher management with their planning, but also to protect your team from being expected to do the impossible. The most important technique for achieving this is known as: guesstimation.

Guesstimating schedules is notoriously difficult but it is helped by two approaches:

make your guesstimates of the simple tasks at the bottom of the work break down structure and look for the longest path through the sequence diagram
use the experience from previous projects to improve your guesstimating skills

The corollary to this is that you should keep records in an easily accessible form of all projects as you do them. Part of your final project review should be to update your personal data base of how long various activities take. Managing this planning phase is vital to your success as a manager.

Some people find guesstimating a difficult concept in that if you have no experience of an activity, how can you make a worthwhile estimate? Let us consider such a problem: how long would it take you to walk all the way to the top of the Eiffel Tower or the Statue of Liberty? Presuming you have never actually tried this (most people take the elevator part of the way), you really have very little to go on. Indeed if you have actually seen one (and only one) of these buildings, think about the other. Your job depends upon this, so think carefully. One idea is to start with the number of steps - guess that if you can. Notice, you do not have to be right, merely reasonable. Next, consider the sort of pace you could maintain while climbing a flight of steps for a long time. Now imagine yourself at the base of a flight of steps you do know, and estimate a) how many steps there are, and b) how long it takes you to climb them (at that steady pace). To complete, apply a little mathematics.

Now examine how confident you are with this estimate. If you won a free flight to Paris or New York and tried it, you would probably (need your head examined) be mildly surprised if you climbed to the top in less than half the estimated time and if it took you more than double you would be mildly annoyed. If it took you less than a tenth the time, or ten times as long, you would extremely surprised/annoyed. In fact, you do not currently believe that that would happen (no really, do you?). The point is that from very little experience of the given problem, you can actually come up with a working estimate - and one which is far better than no estimate at all when it comes to deriving a schedule. Guesstimating does take a little practice, but it is a very useful skill to develop.

There are two practical problems in guesstimation. First, you are simply too optimistic. It is human nature at the beginning of a new project to ignore the difficulties and assume best case scenarii - in producing your estimates (and using those of others) you must inject a little realism. In practice, you should also build-in a little slack to allow yourself some tolerance against mistakes. This is known as defensive scheduling. Also, if you eventually deliver ahead of the agreed schedule, you will be loved.

Second, you will be under pressure from senior management to deliver quickly, especially if the project is being sold competitively. Resist the temptation to rely upon speed as the only selling point. You might, for instance, suggest the criteria of: fewer errors, history of adherence to initial schedules, previous customer satisfaction, "this is how long it takes, so how can you trust the other quotes".