Effort variance is the term used in QA to refer the difference in the Planned effort against Actual effort. Here effort is in the terms of Hours. To calculate the Effort variance we use the following formula :

Actual Effort - Planned Effort
----------------------------------- * 100
Planned effort

Lets take an example where we have estimated the project effort as 100 hrs, But the project is completed in 120 hrs so here the Actual effort is 120 and planned efforts is 100 hrs, so according to formula the effort variance will be :

120 - 100
------------- * 100 == 20 %
100

So effort variance is 20 % .

If Effort variance come out be 0 or negative then it is treated as good effort variance, but if it comes out more then expect then it is not consider as good for the project.