Asymmetric distribution illustrating the positions of its mean, median and mode.
Three important parameters describing an `average' value:
Mean: the average value of an experimental
distribution 
.
Mean of the parent population 
.
The weighted mean is given by 
.
Median is the middle value, 
, defined
so that 
.
Mode is the most probable value, 
, defined so that
).
The mode is often the most useful quantity (eg it gives the true sky level on a frame) but is the hardest to calculate, and depends on the bin size chosen.
There are two forms used for standard deviation, 
, a
characteristic measure of the uncertainty attached to determination of
the mean:
The variance, 
of the mean = 
.
The variance, 
of the sample population = 
.