Provide me the complete answer of the following problem

Problem- Research designs involving control groups require that some employees receive training while others do not. This apparent partiality can be problematic if training improves some employees' performance and not others. If the improved performance leads to promotions, raises, or improved well-being:

Part 1- Is it fair to train some employees and not others just for the purpose of evaluation?

Part 2- What are some of the issues that could be raised as a result of this practice?

I need help to describe the issues that could be raised as a result of this practice.