Project Lead and Lag time

There was a question about the difference between Lead time and Lag time in project Management. The best way I can explain it is ;

Lead time happens when a task should theoretically wait for its depend task to finish, but still you can start little early. So the 1st task and the task you start little early will overlap. So we call it lead time.
Ex: You have to test a program once you finished developing it., But if we start testing a small part of it when bout to finish ., that testing time is the lead time.
Lag time – Lag time is the minimum amount of time that should pass between the finish of one activity and the start of another.
Ex: If there are 2 tasks as
1. Painting a picture
2. Framing it
You cannot start the 2nd task before finishing the 1st one like we did in Lead time and we have a time to wait till we start the 2nd one . (Just think you have to wait 1 day till the painting gets dry. So you cant frame as you finish drawing ) So that waiting time is the Lag time.,

If we are working with PERT we can calculate lead and lag time with numbers. In Software projects, I always come across with lead time.. But lag time happens very rarely in scheduling.. Very simple reason….Because in IT, we cant afford to wait :-)