# Math Help - new mean from old mean

1. ## new mean from old mean

Lets say there's 10 people, each give a rating out of 10 for the new batman movie.The average from the 10 ratings are (sum/10) = 8.7

now one of those 10 changes his rating to a 8, is there are way to calculate the new average without having to go through all the data again?

what about if one of the 10 deleted his vote? so you have 9 votes now and the old average of 8.7?

I assume there is no quick fix solution other than to simply re calculate everything... the problem is that theres 50,000+ votes ..

2. Originally Posted by mymax
Lets say there's 10 people, each give a rating out of 10 for the new batman movie.The average from the 10 ratings are (sum/10) = 8.7

now one of those 10 changes his rating to a 8, is there are way to calculate the new average without having to go through all the data again?

what about if one of the 10 deleted his vote? so you have 9 votes now and the old average of 8.7?

I assume there is no quick fix solution other than to simply re calculate everything... the problem is that theres 50,000+ votes ..
If you know the old rating and the new rating you can form the new mean:

$m_{new}= \frac{Nm_{old}-r_{old}+r_{new}}{N}$

Or if a vote is deleted:

$m_{new}=\frac{Nm_{old}-r_{old}}{N-1}$

The basic idea is that $Nm$ is the sum of all the ratings to be used.

CB

3. Thanks it works great