Calculating average without knowing number of elements

Question

I would like to calculate the average of a a set of numbers without knowing the sum of the set. In other words, I want to calculate the average with knowing as little information as possible.

Note: I do not want to use $\frac{sum}{total}$

I am using looping (induction, I believe) to calculate the average.

I would like to keep track of only one variable, say x, which is equal to the current accuracy.

Is it possible to calculate x without knowing sum?

In summary: seeking how can you calculate the new average knowing only the number of elements and the current average.

Answer 1

Notice that sum is the current average times the current number of elements...

Suppose that $a(n)$ is the average of the first $n$ elements of the set and $e$ is the $n+1^{\text{st}}$ element. Then $$ a(n+1) = \frac{n a(n) + e}{n+1} \text{.} $$ Of course, all this does is reconstruct sum, update it with the new element and then divide to get the new running average.

Answer 2

You need to keep track of two numbers. If you don't you don't know how much a new number changes the average. Say the average of the first bunch is $100$ and you get a new one of $1$. If the first bunch is just one, the new average is $50.5$. If the first batch is $100$ the new average is about $99$.

You can do it in a loop if you keep track of the current average $a$ and the number of items seen so far $n$. This hides the fact that the total so far is $an$. If you get a new value $d$ you can update the average to be $\frac {an+d}{n+1}$ and the count to be $n+1$. In a sense this is doing the same thing, but you don't have to keep the whole list available.

Answer 3

Let us start with an example.

Find the average of $$\{ 3,5,6,2\}$$

$$3+5=8,8/2 = 4$$

$$2(4)+6 =14, 14/3 = 14/3$$

$$3(14/3)+2=16, 16/4=4$$

Which is the final average.