FAQ: Averages
There are three different ideas that people use when the term 'average' is used. Mathematicians use special terms for these:
The Mode
sort all the values into groups and the mode is the biggest group.
The Median
sort all the numbers into order and the median is the middle one.
The Mean
there are several types of mean: in each of these something like a 'divide by the number of values' happens.
Arithmetic Mean
this most often what you may be thinking about when you hear 'average': add the numbers together and divide by how many there are.
Geometric Mean
add the logarithms of the numbers together, divide by how many there are and use the anti-log of that.
Time Weighted
the idea here is that what happens now is twice as important as last time, three times more important as the previous time before that, and so on.