The simple school maths level answer

Never average averages.

The slightly better answer

Never average averages unless the sample sizes of all elements are identical

The reason

suppose you have a set of average speeds for traffic along a road

January 30mph

February 45mph

March 15 mph

What is the average speed for the quarter

You cannot tell as you do not have sufficient information - you might think its (30+45+15)/3 = 30

That is highly unlikely to be correct - for two reasons

1 you don't know the sample sizes

2 speeds are ratios

Now suppose you know that you have

January

Vehicle a 10 miles at 30 mph

Vehicle b 5 miles at 30 mph

vehicle c 25 miles at 30 mph

February

Vehicle d 10 miles at 50 mph

Vehicle e 10 miles at 40 mph

March

vehicle f 25 miles at 15 mph

Total distance travelled = 85 miles

Total time taken (10/30)+(5/30)+(25/30)+(10/50)+(10/40)+(25/15)=

3.45 hours

Average speed= 85/3.45

=~ 24.64 mph