Harvey Motulsky hmotulsky at graphpad.com
Wed Jun 19 17:47:13 EST 1996

Assuming that the number of replicates is the same in the "positive" and
groups, the SD of the difference is the square root of the sum of the
square of the two SD
 SDa-b = sqrt(SDa^2  + SDb^2)

Dr. Harvey Motulsky
President, GraphPad Software
hmotulsky at graphpad.com

> scicluna at ivi.admin.ch (Luke Scicluna) wrote in article
<4q3nk5$res at fwsrva.bfi.admin.ch>...
> Could anyone help me with a question.
> 	My problem is I have a positive result from ELISA and a background  
> result.  I would like to subtract my background result from the positive
> result.  These are two means of an average of two wells.  I would like
> take into account in this calculation the deviation within each mean,
> the positive and negative results, so as to have a more representative  
> result of the variation.  This I would like to do by weighting the  
> resulting value from the subtraction of the two means by the standard  
> deviation (or error of the two means).  Is such a calculation possible.
> 	Is this clear !!!!!! I hope so statistics is not my field.
> 	Thanks for any possible ideas.
> 	Luke Scicluna
> Luke.Scicluna at ivi.admin.ch

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