How fast do skills degrade?

After playing a colony for a long time, colonists with a wide range high level skills in different areas, seem to lose their skill level. At first I thought it was because they were not practicing those skills frequently but skills where degrading at the same rate even though they were being used.

Does anyone know what effects skills degrading, how fast they degrade and what you can do to minimize/slow down the rate of degradation?

EDIT:
An answer has already been given but numbers of how fast it degrades would be additionally helpful. e.g. 20xp loss every day + 3xp every skill level above 10.

Solution 1:

When a skill is above level 10, it can degrade. The speed at which it degrades is determined by the skill level - you lose experience at a faster rate at a higher level.

For example, a level 16 researcher degrades faster than a level 11 researcher. The speed will slow down the lower the level gets.