Is it wrong to say 'above 20 percent of ...'?
I only stated my preference based on what I believe to be a subtle difference between the two prepositions. This would be my favorite sentence:
- Over 20 percent of the employees in the company was above the age of 50.
In Seth Lindstromberg's English Prepositions Explained, both prepositions are correctly presented as meaning "more than":
4.1.1 Above ~ ‘more than’
As a preposition of upness, above participates in expressing the systemic metaphor up is more (Lakoff & Johnson, 1980: 15–16):
(13) The wind never got above Force 3. BNC
4.2.7 Over ~ ‘more than’
This very common sense of over is related to that seen above in §4.1.1 (see also §4.2.8).
The basic idea is that up = more (cf. higher prices). One example should suffice: over a million people = ‘more than a million people’.
Surfing the Internet for more information, I found a dictionary entry that might help account for my preference (the bolds are mine):
2 used for talking about measurements and quantities
Above and over are both used to talk about measurements, for example, when you are talking about a point that is higher than another point on a scale.
Any money earned over that level is taxed.
The temperature rose to just above forty degrees.
Be Careful! Don't use above in front of a number when you are talking about a quantity or number of things or people. For example, don't say `She had above thirty pairs of shoes'. You say `She had over thirty pairs of shoes' or `She had more than thirty pairs of shoes'.
They paid out over 3 million pounds.
He saw more than 800 children, dying of starvation.
Although "above" precedes a percentage, and a singular verb is used, in Above 20 percent of the employees in the company was ... we find the noun "employees," which refers to people, so my impression is that "over" works better there.
And in this other dictionary, a restriction is stated concerning the use of "above" before numbers in general:
We normally use over not above with numbers:
I get over sixty emails a day.
Not: I get above sixty emails a day.
If you weigh over 100 kilograms, then you may need to start a diet.
Not: If you weigh above 100 kilograms
When we talk about temperatures in relation to zero or (the) average, we use above not over:
It was three degrees above zero.
Not: It was three degrees over zero.
When we refer to temperatures in other contexts, we can normally use above or over:
The temperature is already above 30 degrees. (or … over 30 degrees.)