For this purpose, it shouldn't be too hard. You don't need the evaluation results for deciding raises or promotions, and you don't need to compare employees to each other. You just need to determine if the hiring decision was correct. We can do it with a binary score:
0. Oops, shouldn't have hired.
1. AAA would hire again.
I suppose you could make finer distinctions (need to fire this person/we regret hiring but will work on it/meets expectations/exceeds expectations), but my point is that the golden standard for evaluating hiring decisions is whether the hire is adequate or not for the job. I mean, if someone gave you an oracle or a time machine that allowed you to evaluate the employee on the job for a while before hiring them, all your hiring problems would be solved. Now, if you cannot evaluate if someone is good enough for the job, then you have a bigger problem.
0. Oops, shouldn't have hired.
1. AAA would hire again.
I suppose you could make finer distinctions (need to fire this person/we regret hiring but will work on it/meets expectations/exceeds expectations), but my point is that the golden standard for evaluating hiring decisions is whether the hire is adequate or not for the job. I mean, if someone gave you an oracle or a time machine that allowed you to evaluate the employee on the job for a while before hiring them, all your hiring problems would be solved. Now, if you cannot evaluate if someone is good enough for the job, then you have a bigger problem.