Did It Stick?

(1) What was the ultimate purpose of the training? Was it to get the employee "minimally qualified" for a new type of work, or was it to improve competency or performance in key task areas where the employee already performs? This affects what you want to measure in follow-up evaluations.

(2) There are usually other variables involved in employee performance that might need to be "filtered out" (availability of proper equipment, shifting of job assignments due to employee attrition or time off, shop environment, support from management or other employees, etc.).

As I said, difficult, but not impossible, and often worth the extra effort!