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Notation of residuals
Let us look at a list of observations 1 to n of variable Y:
This list is represented algebraically as:
The mean, written out completely, is:
If we represent the deviation of each observation from its mean as yi
In which case :
their sum is Σy2
which makes their mean Σy2 / n
Which is the variance.
Incidentally, the Mean Absolute Deviation (MAD), could be written like this :
Where |y| is the 'absolute' value of y