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Thread: Formula Help!

  1. #1

    Formula Help!



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    Hi I am doing a project in work looking at predicted mortality. This uses a scoring system known as P-POSSUM based on various physiological parameters.

    As per the website riskprediction.org.uk the equation used is:
    Ln R/1-R = -9.065 + (0.1692 x physiological score) + (0.1550 x operative severity score)

    I already have all of the physiological and operative scores.

    Extrapolating R from the above equation using X to represent -9.065 + (0.1692 x physiological score) + (0.1550 x operative severity score)

    Ln R/1-R = X

    R = e^x /( 1 + e^x )

    I have got as far as:
    =(-EXP(X)/(EXP(X)-1))

    For whatever reason I cannot get the same scores with the exact same data as the above website calculator for P-POSSUM
    Is this something to do with rounding errors or is my formula just wrong?! help!
    Thanks in advance
    Chris

  2. #2
    Administrator Ken Puls's Avatar
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    Hi Chris,

    I wish I could help you with this, but unfortunatley my math skills aren't good enough to be able to tell if your interpretation is correct or not. (Been 15+years since I've worked with math at that level!)
    Ken Puls, FCPA, FCMA, MS MVP (Excel)

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  3. #3
    Look at the formula documentation. Maybe there are known "issues" that may affect the result.

  4. #4
    Chris,


    don't know if this would help you or not but here is a little different approach. use a formula to solve for your mortality based on below. say in cell A1 you solve for x then you write your formula to equate mortality% based around that. you say you already have your Phys Scores and Op Scores. I will assume you enter them in cells like c1 and c2 (or somewhere). in A1 write your formula to solve for x

    X= (0.1692 x physiological score) + (0.1550 x operative severity score)



    now put a formula in say A2 to solve the equation below. this will give you your mortality %

    1/(1+ e-x)


    Example:
    in A2 enter this formula.  
     
    =SUM(1/(EXP(-A1)+1))*100


    here is a file that uses this same approach. you can test the results against the website you referenced.
    Attached Files Attached Files

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