How do I measure variability / volatility of a set of numbers?

[Phedwards]

I thought Standard Deviation measured the extent of variability of a set of numbers, but the following sets of numbers both have the same degree of variability, but different Standard Deviations. So what measure should I use?
STDEVP(1, 3, 7, 4) = 2.17
STDEVP(10, 30, 70, 40) = 21.65

[shg]

If you normalize by dividing by the average value, he deviations are the same:

=STDEVP(1,3,7,4) / AVERAGE(1,3,7,4) = STDEVP(10,30,70,40) / AVERAGE(10,30,70,40) = 0.577...

Look at how deviation is calculated.

[Phedwards]

thanks - I've added that in to my formula!