Q:

The difference between for and over in terms of time

This is my first question. So please go easy on me if possible. 
I can't understand the difference between 'for' and 'over' in terms of time.
For example, What is this difference: 'The birth rate has been increasing over ten years', and 'The birth rate has been increasing for ten years'.
If I can give you another example, what is the difference between 'for centuries' and 'over centuries'.
Maybe this is a very easy question but I can't find out the difference. Please help me.

A:

The birth rate has been increasing over ten years means there may be minor ups & downs when observed in a smaller time window, say an year, but over the span of ten years, there is definitely an increase in birth rate.
The birth rate has been increasing for ten years means there is always an increase in birth rate no matter how small the time window is, in the ten years. In mathematics, this is considered as a strictly increasing function of time.

