Hi Kabamaru,
Section 3.3 of the in-game help explains how stars are calculated, I have pasted it here for your reference. In your example: a doctor can not be trained, so his level will always stay the same, and you will normally level up regularly, so his stars will only go down after time, never up.
3.3. Star ratings and skill levels
Stars indicate skill relative your current level. So, if you're currently level 10, a level 10 driver or staff would be displayed as 5 stars. A driver or staff that's half your level will have 2.5 stars and so on. This is why, as you progress through the game and level up, you may see the star rating of your own drivers and staff decrease. Their skill level has not actually changed, but your level as a manager has increased relative to them and you are now in a position to get higher level drivers and staff. The star rating acts as a clear way of indicating this to a manager.
If you ever see red stars on drivers or staff, then they have a higher level than you, and you would not be able to hire them. It's possible through driver training to raise your driver(s) levels above your manager level, but training becomes very slow beyond that.
Guest