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I'm just beginning and despite my search I cannot find any simple formula that would return a number between 0 and 100%, i.e. I do not care whether there was over or underestimation. The simplest thing I could think of would be:
Actual Effort - |Estimation Delta| / Actual Effort.

To me, using simply Estimation Effort / Actual Effort can give more than 100% in case of overestimation, and Accuracy 125% sounds weird.

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If you're looking for any function that return the value inside 0-100 it makes sense to check Sigmoid function. It produces values from 0 to 1, which are easily converted to 0 - 100;

This is my example on how to use Sigmoid approach.

Here is the normal Sigmoid function:

enter image description here

You have to choose the acceptance range start point starting from which you consider your accuracy value equal to 100%. Let it be 4. Zero value would be -4. However your estimation is not 4 but 10 as you point out in your comment below. Thus you need to convert the coordinates where your 10 would become 4 for Sigmoid and 0 would become -4. This is the simple linear expression. It would look like:

d(x) = 4/5 * x - 4, where x - your estimation. Here is it on the chart.

enter image description here

Now what you only need is the absolute value of deviation from the estimated value. Say it would be diff and according to your comment below the actual work is measured as 200 units of whatever. Then you calculate:

Accuracy = Sigmoid(d(10 - |diff|)) * 100,

hence if your diff = +/-5, your accuracy would be 50%, if your diff = +/-190, your accuracy would be zero.

And here is the resulting graph for your case:

enter image description here

  • But this formula does not really make sense: if you estimate 10, and observed 200, the formula result is accuracy of 95%! And if your estimate is entirely correct, then VA-VO=0 and the whole result will be 0. – John V Feb 8 '18 at 11:06
  • @user970696 check my update on Sigmoid approach – Alexey R. Feb 8 '18 at 13:09
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Estimation accuracy formula

You said:

I cannot find any simple (estimation accuracy) formula that would return a number between 0 and 100%

  1. When the actual is the same as estimation, we want the estimation accuracy to be 100%.
  2. When the actual is 0 we want the estimation accuracy to be 0%.
  3. When the actual is twice the estimation (or more) we want the estimation accuracy to be 0%.

Given these, the formula would be: (Estimated - ABS(Estimated - Actual or 2*Estimated whichever is less)) / Estimated

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