Say I have values
10, 3, 6
What would be a way of finding a subjective difference between them. Or in other words, how close they are to each-other.
So if i had
3, 2.5, 2.8; they would be close together in relation to one another.
3, 10, 10.5; would not be close together, despite two values being similar.
Compute the standard deviation of the set.
In other words, let $m = \frac{1}{n} \sum_{i=1}^n x_i$ be the average of the set, then compute $$ \sigma = \sqrt{\frac{1}{n} \sum_{i=1}^n \left(x_i-m\right)^2} = \sqrt{\frac{1}{n} \sum_{i=1}^n x_i^2 - m^2} $$
It's nice that this will work for $n=1,2$ or even very large $n$.
UPDATE (thanks to BlueRaja - Danny Pflughoeft)
This is will handle a absolute difference reasonably. However, if you want a relative difference (e.g. $90,100,110$ should behave similar to $9,10,11$) - compute the relative standard deviation, dividing by the mean or the median of the set, i.e. $\sigma/m$.