I know it's possible to calculate a regular average incrementally knowing the previous average, the next number and the number of items. But is this possible for a moving average of N items, at least approximately?
To calculate the exact SMA in this case you must know the first number in a set which you're about to leave out, as shown here. That's why I ask about the approximation.
I tried an approach similar to the one for a regular average: multiply the previous average by N - 1, add a new number and then divide by N. But the result looked like garbage to me.