Dear Smita,
There is a discrepancy in the heading of your post and the post itself. By reading the heading, one would interpret that the query is on measurement of the variability of the performance. On the contrary, the body of the post talks about eligibility for the payment of PLI.
Let me believe that PLI stands for Performance Linked Incentive (PLI). As far as payment of PLI is concerned, it is the internal matter of your company. Payment will depend on the terms of employment mentioned in the appointment letter.
If employee has resigned without completing even six months then what performance he/she has delivered? Possibly, those who leave well before completion of six months, for them I doubt whether you recover their salary also. Then where is the question of payment of PLI?
The confusion has arisen out of not having proper policy on payment of PLI. First define the eligibility criteria and include it in the policy.
Thanks,
Dinesh Divekar