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Suppose you are a project manager. You can estimate an effort in days for specific task for specific developer. After performing estimation you obtain some min and max values.

After this you delegate a task to developer. Actually you also set up deadline.
Which estimation is better to use when set up deadline: min or max?

As I see min estimation can result in stress for developer, max estimation can result in using all the time which is allocated to developer even if task can be complete faster (so called Student syndrome). Which other pros and cons of two approaches?


Small clarification: I speak about setting up deadlines for subordinates when delegating the task, NOT for reporting to my boss.


To add one more clarification: I can keep in mind my real estimation, provide to boss slightly larger estimation, to subordinates - slightly smaller. And this questions touches the following thing: is it good idea to provide to developer underestimation to make him working harder?

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What about the average of min and max? – innaM Nov 30 '09 at 12:33
@innaM you can find this in ChrisF answer – sergtk May 22 '10 at 23:03

12 Answers 12

You should use the best guess which is a function of the min and max estimates* - not just the simple average -

best_guess = (min * min_weighting + max * max_weighting) / divisor*

* Tom Neyland suggests it should be (min_weighting + max_weighting). Actually I'm not sure whether that is correct, but it's probably more correct than my original divisor of 2.0.

The weighting you give to the min and max values will depend on the complexity of the task, the risks associated with the task, the likelihood of the risks occuring, the skill of the developer, etc. and will vary from organisation to organisation and from project to project. If you keep a record of your previous estimates and the actual time each took you'll be able to refine these estimates over time.

You should also use these values, plus a confidence value, when talking to senior management and customers. While giving the max and delivering early is not the same as giving the min and delivering late, it still shows that you don't have control over your development.

Giving the confidence value and an idea of the risks will also help manage expectations so if there are problems they're not unexpected.

* These min and max estimates will be got by various means - asking the developers, past experience etc. If polling developers then the actual min and max values should be treated as outliers and either discarded or modified in some way. What I mean here are the values you get from phrases like "it'll take 2 weeks if all goes well or a month if we hit some snags". So the values you plug into the formula are not the raw numbers.

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+1, very nice answer – rahul Nov 30 '09 at 12:35
+1 thanks! good ideas – sergtk Nov 30 '09 at 12:37
Is that first parenthesis in the right place? – gnovice Nov 30 '09 at 18:26
@gnovice - thanks! – ChrisF Nov 30 '09 at 20:59
Should it be (min...max_weighting) / (min_weighting+max_weighting)? – Tom Neyland Apr 8 '10 at 16:56

Use neither min nor max but something in between.

Erring on the side of overestimation is better. It has much nicer cost behavior in the long term.

  • To overcome the stress due to underestimation, people may take shortcuts that are not beneficial in the long term. For example, taking extra technical debt thast has to be paid back eventually, and it comes back with an interest. The costs grow exponentially.

  • The extra cost from inefficiency due to student's syndrome behaves linearly.

Estimates and targets are different. You (or your managers and customers) set the targets you need to achieve. Estimates tell you how likely you are to meet those targets. Deadline is one sort of target. The deadline you choose depends on what kind of confidence level (risk of not meeting the deadline) you are willing to accept. P50 (0.5 probability of meeting the deadline) is commonplace. Sometimes you may want to schedule with P80 or some other confidence level. Note that the probability curve is a long-tailed one and the more confidence you want, the longer you will need to allocate time for the project.

Overall, I wouldn't spend too much time tracking individual tasks. With P50 targets half of them will be late in any case. What matters most is how the aggregate behaves. When composing individual tasks estimates into an aggregate, neither min or max is sensible. It's extremely unlikely that either all tasks complete with minimum time (most likely something like P10 time) or maximum time (e.g. P90 time): for n P10/P90 tasks the probability is 0.1^n.

PERT has some techniques for coming up with reasonable task duration probability distributions and aggregating them to larger wholes. I won't go into the math here. Here's some pointer for further reading:

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Thanks for the wide answer and links. Actually the initially this question appeared when I find that McConnell in "Software Estimation..." suggest to overestimate than underestimate, but Malik in "Managing, performing, living..." suggest to underestimate. – sergtk Nov 30 '09 at 21:29

Ask for best, likely and worst case scenario estimates instead. Then use Program Evaluation and Review Technique. However you may want to take a look at some PERT critique first.

For individual tasks or tasks making up the critical path it’s simply not prudent to go for the best case estimates. It’s like saying that the project is absolutely free of any risk and uncertainty. If the actual job turns out to be anything but the best case scenario you’ll end up blowing the schedule. It’s better to end up with some extra time on your hands and fill the time by implementing some nice-to-haves as opposed to having to work nights and weekends.

On the other hand if managers mostly went for the worst case estimates and in software world they can easily be an order of magnitude greater than the best case figures most projects would never make it past the feasibility and planning stage. Not all of the risks going to materialise.

Going for the best case estimate won't help fighting student syndrome. Include interim milestones and deliverables instead, beside being helpful at combating the student syndrome they're pre-requisite for having a trustworthy data on the project progress and uncovering early any potential issues.

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We use PERT in our projects, given that the skill levels in our team are quite varied. To developers, overestimation is always better than underestimation. But in the long run, you should be able to gather "just enough" information to create a more justifiable schedule. – geffchang Apr 9 '10 at 14:51

If the difference between min and max are big rather than using some black magic formula I think it the best thing to do would be to go back to the developers and ask them to do a finer breakdown and prototyping, which will lead to better estimates where the gap between min and max is not that big.

Note to the question: In my opinion, the estimates should be done by the developers/architects since they have the best technical knowledge to be able to break down into tasks and estimate those tasks.

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yes, I agree that developers should be involved. I didn't added this into the question to simplify it. – sergtk Nov 30 '09 at 12:45

If you are estimating for a specific developer, and you know your estimates are generally accurate for that developer, then the min value is the logical deadline (initially). In the course of the project you will adjust deadlines according to circumstance.

If you have little experience with a specific developer, one of my fondly regarded previous managers would ask the developer himself to do the estimate and set the initial deadline a third of the distance between that developer's min and max, challenging the developer to beat it.

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Something which has been missing in many of these answers (perhaps because it's slightly off-topic) is frequent updates. With younger/newer developers this is even more important - read the code they commit, and/or check in daily to ask them for specific, detailed reports.

This also allows you to set tight deadlines for developers without giving them too much stress, because they will know you're around to help adjust deadlines when needed.

Frequent updates give you the most important tool in setting customer/management expectations - early warning of issues which might delay things, and I prefer having that over any formula.

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Is the developer going back into a cave to develop this or is there a good chance of changing requirements over the course of the project? I would think most projects will have a good chance that something won't go smoothly and thus it may be better to try to get the prototype up sooner rather than later.

As for the initial question, I think I'd break this out into a few different outcomes and consider each:

Gross underestimation -> This leads to the problem that there is still a lot of work to do and the manager appears unable to make reasonable estimations.

Minor underestimation -> In this case, either there is an extension, scope gets cut or some bugs are in the release, but this is better than the previous case.

Made the deadline, on time and on budget with quality -> While this may seem optimal as everything worked out, I don't think this is the best result possible.

Minor overestimation -> In this case, there is some breathing room that means either things finish early or some extra work is added. A point here is that this may seem to deliver a slightly better result than the previous case like how some companies will try to beat the earnings estimate by a small amount to do better than expected.

Gross overestimation -> I think this would be the worst case outcome though it is similar to the first in terms of someone being way out of their league in being able to provide a reasonable estimation.

That's just my opinion on each and others may have a different take on it than me.

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If you're trying to hold developers to their minimum estimate, that's foolish. No one, in any industry, consistently hits their minimum time estimate for getting something done. Eventually, they'll just learn to pad their minimum estimates significantly, and then they'll never hit the old minimums, because all estimates will be above that.

In Agile/Scrum, you don't set firm deadlines, but set "how many hours left on this task". Every day, you update the amount of time left. You do not track hours spent, but do track estimated hours remaining, and you try and stay honest about it.

If you have lazy developers, this is bad, because they can easily game that system. If you have developers that are worth their salt, this is great. They get better at estimation pretty quickly, and you - as a project manager - learn how reliable their estimates are, and you'll have a much better feel for what estimates to pass up the chain based on the individual developer estimates.

Go slightly towards Agile, fire the bad developers as you discover which are which, reward the good developers for actually giving a damn, and have a more productive, happier team while being able to report more accurate expectations to your superiors.

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Again, I'll stress that "going for the minimum estimate" will just get the developers to lie to you, knowing that otherwise, they're putting themselves into an awful situation everyday. – Dean J Apr 9 '10 at 14:34
I mean that it is possible that developers works better under time pressure, but stress can influence negatively. Actually now I have situation when developer under-estimate the task, from my experience I see that this is underestimation and report to my boss much higher estimation, but if I say about this to my subordinate he can relax because he can think that I am stupid :) and provade a lot of time for him – sergtk Apr 9 '10 at 14:42
@sergdev: What I've seen suggests that developers usually have enough internal pressure that pushing them harder is generally counter-productive. BTW, your developers aren't dumb (if they are, you need to fire them), and they will catch on to the estimate juggling you do. You're probably better off doing it openly than trying to keep it secret. – David Thornley Apr 9 '10 at 14:57
@David Thornley Frequently it is not a secret, I ask to developer: are you sure in your estimation? He say "yes", and I don't change his estimation and afterward I teach such developer: I say him that he should analyze better in future, pay attention to the sources of errors. But my point concerns responsibility: I am responsible and try to make correct estimation to my customers and my boss despite how good in this my subordinate currently. – sergtk Apr 10 '10 at 5:28
@sergdev: If your developers think that you're dumb, it's time to work on replacing them with developers who want to get the job done. If the developers you have now are smart, they know they're allowed to miss the deadlines you're telling them, and they're going to game the system that way. If they're not smart, why did ya hire 'em? :-) Again, it's easiest to have the developers actively managing their own estimates as much as possible, because then they have the greatest motivation to improve their estimates over time. – Dean J Apr 12 '10 at 12:41

If in doubt under promise and over deliver: you want to be the person who is delivering more than they were expecting, not less. Based on this always go with the higher of any estimate.

Slightly more complex:

For a given potential delivery, if you plot the delivery times against the chances of them being happening, you're going to get a curve which is a variation of a normal distribution, and you can assume that a developers minimum estimates are going to be somewhere towards the left of the curve and their maximum towards the right.

The area under the curve to the left of the single number you select as your estimate represents the probability of you successfully delivering on or before that estimate. So if you give a number at the very left hand side your chance of hitting is effectively zero, if you give a number at the very right hand side your chance is effectively 100%.

What is less commonly realised is if you give the mean value (assuming your min and max averaged out give something approximating the actual mean) you'll only hit that deadline 50% of the time. Effectively if you use the mean you're going to miss the deadline half the time. I don't know about you but I don't like being seen as the guy whose misses half his deadlines.

So you want a number which is going to give you something you hit, say, 90% of the time. Conveniently 95% represents the mean + two standard deviations but if you can't be arsed to calculate that (and most of us probably don't have the data) my experience says that:

(3 x max + 1 x min) / 4

gives a reasonable result.

Incidentally, what you tell the developer is the deadline is another question entirely. Personally I'd give him somewhere around ((2 x max + 1 x min) / 3) and have the rest as contingency.

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What are you using the estimates for? Specifically, why will the developer feel stressed if you normally underestimate?

If you're trying to schedule how long something is likely to take, you go for an intermediate value. Probably on the long side, since people normally underestimate. In any case, you shouldn't be using these estimates as firm objectives for developers, and so they shouldn't be overly stressful.

If you're using these estimates to set up commitments, you need to err on the side of overestimating. Giving developers insufficient time leads to burnout, unmaintainable buggy code that doesn't do quite what the user wants, and low morale and high turnover. Set the commitments to be reachable, and encourage the developers to finish early.

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This depends on project.

Some projects may require fast development and there's no alternatives if deadline is already set and there's no good chance to prolong development. Typical issue: marketing campaign resulting in new service. Such deadline can be enough for normal development, but in some organizations it is so close, that developers work in stress and make many errors that are fixed during production stage. That's a kind of project when developers have to work with topmost effectiveness and they'd better get good reward on success.

Some projects are accurately planned and here you can use all analytics you have: history data, some developer's time metrics on subtasks, calculating risks, etc.

But anyway MAX time shouldn't be used: its the most inaccurate measure that usually leads to even more time taken. And here's a simple reason: when developer just gives away this MAX, he almost doesn't measure. He just gives away his intuition that has very little info at the time. But if he'll spend at least half an hour he'll understand specifics of his tasks, he even may split it into subtask and increase his accuracy. So you can give developer some bias like "hey, guys, just think in what time you would provide stable code here" but send him measure himself. It is good for a job, it is good for a programmer himself.

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The first mistake most estimators use when setting the deadline is assuming that the dev will be full-time every day on that task which is a disastrous mistake. This can result in not meeting the deadline even when you use the over estimate to figure out the deadline. Being under the hours but past the deadline you told the client is a big problem. People take leave, get sick, have jury duty, have to go to required meetings on some new HR policy, get called over to help on another project when someone is stuck, have to load software on a new computer when their old one breaks, have to research a production problem on code they recently deployed, etc. If you are estimating more than 6 hours a day on the project per person, you are already in trouble on the deadline before the project starts. When I did manpower studies, we used a figure that equated to just slightly more than 6 hours a day of direct work when calculating out how many people were needed for any job. And we did a lot of statistical analysis as the basis for the figure we used.

I think you have to decide which of these to use on a case by case basis. We have some projects that we know the max estimate is still probably a little low (usually when someone in management couldn't face the client with the real estimate), we have others where we are doing something new where we know the estimates are more likely to be off, in these kinds of cases go with the max. But for work you've done before that is well-defined and you know the dev assigned won't be learning new skills, then go closer to the min (but never actually use the min, there are alawys unexpected bumps in the road). ALso the shorter the project, the more likely you will be able to meet the min, it is far easier to get a good estimate for a week-long project than a year-long one.

More importantly is changing the estimate and deadline every time the circumstances change. If the client adds work, the extend the deadline and estimate, don't just do it. If your best dev quits and you have to put someone new on the project, extend the deadline becasue that person has to have time to get up to speed (you may have to eat the hours though, the client may not agree to pay for that time. Critical to this is telling the client right away. They tend to be better about moving a deadline (although not happy) than they are about missing one or making the dealine but the product doesn't work as they expect it to. Too many project managers just like to wish a problem is going away and the won't have to face that conversation with the client. But usually when they do finally have to tell him it is a much worse conversation than the difficult one they tried to avoid.

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