During staff evaluation, which is more effective: focusing on employee weaknesses or his strengths? Why?

weaknesses.

By doing so, you are able to understand and improve the weaknesses and finding a way to turn them into strengths.

Build a trust and deal fairly with each employee, not allowing favoritism or personality difference to affect judgement. When a worker trusts their boss, they are motivated to greater productivity, achievement and loyalty.

As a previous trainer, I don't believe on a person having a weakness or a strength. Each person is capable of doing what others can. Other people who think some of their traits are what we call weaknesses, are just traits that are undeveloped due to influence of the society and their environment. Focusing on weaknesses only when it comes to staff evaluation will hinder you to see the person's full potential. You should look at the person as a whole -- his traits (both weaknesses or strengths as others say), how he works with others or his working attitude. By then, you should see his full potential when you evaluate them and it can also be beneficial for this person and the company as well.

Focusing in employee's weakness. Weakness makes them slow and less effective so we should motivate them. Focus on their weak side, teach and motivate them so that they will be a better or yet the best employee. A good employer starts from a good employee.