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Math Help - new mean from old mean

  1. #1
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    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 ..
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  2. #2
    Grand Panjandrum
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    Quote Originally Posted by mymax View Post
    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
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  3. #3
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    Thanks it works great
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