Formula Help!

drchrisbrown87

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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^[SUP]x[/SUP] /( 1 + e^[SUP]x[/SUP] )

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
 

Ken Puls

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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!) :(
 

Mary

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

tommyt61

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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 %

[SUP]1/(1+ e[SUP]-x[/SUP])


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

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

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