Effort Estimation Variance
When estimating effort it is important to keep track of the accuracy by comparing against the actual outcome. This KPI will represent which is the variance between the estimation and the actual effort for each iteration. This number will allow you to start searching for patterns that could explain the variance and correct it.
Download this Chart

Hints!

Estimation
The initial effort estimation for the iteration. This could be expressed in the most suitable form such as user story points or task hours. 
Actual Effort
Once the iteration is done we can answer the question: "Which was the actual effort?" 
Variance
Variance should tend to zero for the iterations to come. If that is not the case, a Sprint retrospective could be a suitable moment to address it. 
Now go and try it!