Removing negative trait from staff

How can you remove negative trait? I know that if you send your employee to training and has a negative trait there is a possibility to lose it. Is it random or you need to have a certain condition (for example a maximum level staff of the basic skill has to be more than 50%)?

Buy Hospital Services DLC, put staff into training room. Each learning cycle there is a chance - maybe 10%, they will lose a negative trait. Currently, the only trait I could not get rid of was dirty feet.
I had staff losing the trait after the first cycle, while others could train 10 times and still have it. Remember, the more the person already knows, the less probable this makes removing a trait as you cannot train people with stats at 100%.