Estimating

Quick! How long will it take to send War and Peace over a 56k modem line? How much disk space will you need for a million names and addresses? How long does a 1,000-byte block take to pass through a router? How many months will it take to deliver your project?

At one level, these are all meaningless questionsthey are all missing information. And yet they can all be answered , as long as you are comfortable estimating. And, in the process of producing an estimate, you'll come to understand more about the world your programs inhabit.

By learning to estimate, and by developing this skill to the point where you have an intuitive feel for the magnitudes of things, you will be able to show an apparent magical ability to determine their feasibility. When someone says "we'll send the backup over an ISDN line to the central site," you'll be able to know intuitively whether this is practical. When you're coding, you'll be able to know which subsystems need optimizing and which ones can be left alone.

Tip 18

Estimate to Avoid Surprises

As a bonus, at the end of this section we'll reveal the single correct answer to give whenever anyone asks you for an estimate.

How Accurate Is Accurate Enough?

To some extent, all answers are estimates. It's just that some are more accurate than others. So the first question you have to ask yourself when someone asks you for an estimate is the context in which your answer will be taken. Do they need high accuracy, or are they looking for a ballpark figure?

If your grandmother asks when you will arrive , she's probably wondering whether to make you lunch or dinner. On the other hand, a diver trapped underwater and running out of air is probably interested in an answer down to the second.
What's the value of p? If you're wondering how much edging to buy to put around a circular flower bed, then "3" is probably good enough. ^[10] If you're in school, then maybe "22/7" is a good approximation . If you're in NASA, then maybe 12 decimal places will do.

^[10] "3" is also apparently good enough if you are a legislator. In 1897, Indiana State Legislature House Bill No. 246 attempted to decree that henceforth p should have the value of "3". The Bill was tabled indefinitely at its second reading when a mathematics professor pointed out that their powers did not quite extend to passing laws of nature.

One of the interesting things about estimating is that the units you use make a difference in the interpretation of the result. If you say that something will take about 130 working days, then people will be expecting it to come in pretty close. However, if you say "Oh, about six months," then they know to look for it any time between five and seven months from now. Both numbers represent the same duration, but "130 days" probably implies a higher degree of accuracy than you feel. We recommend that you scale time estimates as follows :

Duration	Quote estimate in
1-15 days	days
3-8 weeks	weeks
8-30 weeks	months
30+ weeks	think hard before giving an estimate

So, if after doing all the necessary work, you decide that a project will take 125 working days (25 weeks), you might want to deliver an estimate of "about six months."

The same concepts apply to estimates of any quantity: choose the units of your answer to reflect the accuracy you intend to convey .

Where Do Estimates Come From?

All estimates are based on models of the problem. But before we get too deeply into the techniques of building models, we have to mention a basic estimating trick that always gives good answers: ask someone who's already done it. Before you get too committed to model building, cast around for someone who's been in a similar situation in the past.

See how their problem got solved . It's unlikely you'll ever find an exact match, but you'd be surprised how many times you can successfully draw on other's experiences.

Understand What's Being Asked

The first part of any estimation exercise is building an understanding of what's being asked. As well as the accuracy issues discussed above, you need to have a grasp of the scope of the domain. Often this is implicit in the question, but you need to make it a habit to think about the scope before starting to guess. Often, the scope you choose will form part of the answer you give: "Assuming there are no traffic accidents and there's gas in the car, I should be there in 20 minutes."

Build a Model of the System

This is the fun part of estimating. From your understanding of the question being asked, build a rough and ready bare-bones mental model. If you're estimating response times, your model may involve a server and some kind of arriving traffic. For a project, the model may be the steps that your organization uses during development, along with a very rough picture of how the system might be implemented.

Model building can be both creative and useful in the long term . Often, the process of building the model leads to discoveries of underlying patterns and processes that weren't apparent on the surface. You may even want to reexamine the original question: "You asked for an estimate to do X. However, it looks like Y, a variant of X, could be done in about half the time, and you lose only one feature."

Building the model introduces inaccuracies into the estimating process. This is inevitable, and also beneficial. You are trading off model simplicity for accuracy. Doubling the effort on the model may give you only a slight increase in accuracy. Your experience will tell you when to stop refining.

Break the Model into Components

Once you have a model, you can decompose it into components. You'll need to discover the mathematical rules that describe how these components interact. Sometimes a component contributes a single value that is added into the result. Some components may supply multiplying factors, while others may be more complicated (such as those that simulate the arrival of traffic at a node).

You'll find that each component will typically have parameters that affect how it contributes to the overall model. At this stage, simply identify each parameter.

Give Each Parameter a Value

Once you have the parameters broken out, you can go through and assign each one a value. You expect to introduce some errors in this step. The trick is to work out which parameters have the most impact on the result, and concentrate on getting them about right. Typically, parameters whose values are added into a result are less significant than those that are multiplied or divided. Doubling a line speed may double the amount of data received in an hour , while adding a 5 ms transit delay will have no noticeable effect.

You should have a justifiable way of calculating these critical parameters. For the queuing example, you might want to measure the actual transaction arrival rate of the existing system, or find a similar system to measure. Similarly, you could measure the current time taken to serve a request, or come up with an estimate using the techniques described in this section. In fact, you'll often find yourself basing an estimate on other subestimates. This is where your largest errors will creep in.

Calculate the Answers

Only in the simplest of cases will an estimate have a single answer. You might be happy to say "I can walk five cross-town blocks in 15 minutes." However, as the systems get more complex, you'll want to hedge your answers. Run multiple calculations, varying the values of the critical parameters, until you work out which ones really drive the model. A spreadsheet can be a big help. Then couch your answer in terms of these parameters. "The response time is roughly three quarters of a second if the system has a SCSI bus and 64MB memory, and one second with 48MB memory." (Notice how "three quarters of a second" conveys a different feeling of accuracy than 750 ms.)

During the calculation phase, you may start getting answers that seem strange . Don't be too quick to dismiss them. If your arithmetic is correct, your understanding of the problem or your model is probably wrong. This is valuable information.

Keep Track of Your Estimating Prowess

We think it's a great idea to record your estimates so you can see how close you were. If an overall estimate involved calculating subestimates, keep track of these as well. Often you'll find your estimates are pretty goodin fact, after a while, you'll come to expect this.

When an estimate turns out wrong, don't just shrug and walk away. Find out why it differed from your guess. Maybe you chose some parameters that didn't match the reality of the problem. Maybe your model was wrong. Whatever the reason, take some time to uncover what happened . If you do, your next estimate will be better.

Estimating Project Schedules

The normal rules of estimating can break down in the face of the complexities and vagaries of a sizable application development. We find that often the only way to determine the timetable for a project is by gaining experience on that same project. This needn't be a paradox if you practice incremental development, repeating the following steps.

Check requirements
Analyze risk
Design, implement, integrate
Validate with the users

Initially, you may have only a vague idea of how many iterations will be required, or how long they may be. Some methods require you to nail this down as part of the initial plan, but for all but the most trivial of projects this is a mistake. Unless you are doing an application similar to a previous one, with the same team and the same technology, you'd just be guessing.

So you complete the coding and testing of the initial functionality and mark this as the end of the first increment. Based on that experience, you can refine your initial guess on the number of iterations and what can be included in each. The refinement gets better and better each time, and confidence in the schedule grows along with it.

Tip 19

Iterate the Schedule with the Code

This may not be popular with management, who typically want a single, hard-and-fast number before the project even starts. You'll have to help them understand that the team, their productivity, and the environment will determine the schedule. By formalizing this, and refining the schedule as part of each iteration, you'll be giving them the most accurate scheduling estimates you can.

What to Say When Asked for an Estimate

You say " I'll get back to you. "

You almost always get better results if you slow the process down and spend some time going through the steps we describe in this section. Estimates given at the coffee machine will (like the coffee) come back to haunt you.

Related sections include:

Algorithm Speed

Challenges

Start keeping a log of your estimates. For each, track how accurate you turned out to be. If your error was greater than 50%, try to find out where your estimate went wrong.

Exercises

9.	You are asked "Which has a higher bandwidth: a 1Mbps communications line or a person walking between two computers with a full 4GB tape in their pocket?" What constraints will you put on your answer to ensure that the scope of your response is correct? (For example, you might say that the time taken to access the tape is ignored.)
10.	So, which has the higher bandwidth?