Duncan,
this type of question applies for other component types, too.
Here is a general answer how to find out:
Type in a value into the data field, click the Pi-factors button and note the failure rate-predicted. Close the Pi-Factors window, change the value in the data field, and click Pi-Factor button again. Look at failure rate-predicted: Did it change? Try these steps several times with different values. If the failure rate-predicted doesn't change at all, then the parameter is irrelevant.
Thomas
