How Long Will It Take?

Hi, it’s Allison again. In this clip, I’m going to teach you an easy and simple way to estimate tasks that you can use right away. Estimating how long it will take to accomplish a task can be surprisingly difficult.

An endless variety of factors and variables can affect the outcome, making an apparently complicated task much simpler than initially expected, or turning a seemingly easy task into a nightmare. But however difficult it may be, estimation is a necessary part of your work. You need good, accurate estimates of task durations in order to build a project schedule and ensure that you deliver your product on time. Coming up with a good estimate is a common challenge that many people struggle with, so, I’m going to teach a simple estimation formula called PERT. Also known as the Program Evaluation and Review Technique, PERT isn’t just a cute name – it’s a vital tool for estimating task duration.

The formula goes like this: Multiply the most likely amount of time that it will take to finish the task by 4 (this is the time that makes most sense, taking into account that you’ll encounter a few obstacles on the way). Then add your ‘best case scenario’ time (this is the fastest that you can complete the task if you have everything that you need, and everything goes according to plan). Now add your ‘worst case scenario’ time (this is the longest time that it will take you to fix the task in case things go wrong). Sum it all up and then divide by 6. The resulting figure is your ultimate estimate for the duration of the task.

Now let me give you an example of PERT in action. Let’s say I want to walk down to the corner market to buy a quart of milk. From my experience, I think this task will most likely take 15 minutes. 10 minutes of walking time, 1 minute to get the item, and 4 minutes to wait in line. So 15 is my Most Likely number. If there is no line, and I walk fast, then I think it will take a total of 8 minutes, so that’s my Best Case number. Now for my Worst Case number I have to consider what might go wrong. And a lot might go wrong. The sidewalk could be crowded, I might nearly get run over by a taxi crossing the street and have to stop to catch my breath, and, worst of all, the line could be three times as long as usual because everyone is stocking up on snacks for the big game tonight. I figure that puts my Worst Case number at 28 minutes.

Now I take the formula: [best case + (4 X Most likely) + worst case] / 6. I plug those numbers in: [8 + (4 x 15) + 28] / 6
And then I get out my calculator. Or, if I’m feeling up to it, I just do the math in my head. My result is 16, and that’s my estimate for the duration of this particular task.

PERT is effective because, even though it gives much more weight to your most likely estimate, it also takes the best and worst case possibilities into account. You will find that it truly makes the difficult job of estimating much easier.

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