Date:         Tue, 30 Oct 2001 13:38:44 -0800
Reply-To:     Dale McLerran <dmclerra@MY-DEJA.COM>
Sender:       "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From:         Dale McLerran <dmclerra@MY-DEJA.COM>
Subject:      Re: Area Under Curve (AUC)
Comments: To: mbarish@progenics.com
Content-Type: text/plain

Melanie,

You could consider an alternative method in which you generate and
operate on a translated curve as follows:

  1) determine the minimum value anywhere along the curve
  2) subtract the minimum from all values.  This subtracts a
     rectangular area of (minimum*length of X axis) from the total
     area.
  3) compute the area under the curve for the translated curve using
     the trapezoidal rule
  4) add back the rectangular area of (minimum*length of X axis)

If the minimum is a negative value, then subtraction of a constant
negative value in step 2 was equivalent to adding rectangular area
equal to (|minimum|*total time length), and the operation in step 4
results in subtracting this rectangular area.  This should be an
easier approach to computing the area than the approach which you
mention below.

Dale


>Date:         Tue, 30 Oct 2001 15:33:31 -0500
>Reply-To: Melanie Barish <mbarish@PROGENICS.COM>
> Melanie Barish <mbarish@PROGENICS.COM>      Area Under Curve (AUC) SAS-L@LISTSERV.UGA.EDU
>Hello everyone.  I've been asked to calculate area under the curve for a
>graph of percent change from baseline versus time at blood draw.  I've
>been doing research in the listserv archives, and I have found some very
>helpful info regarding the trapezoidal method of calculating AUC.
>However, in this case, because my graph is of percent change from
>baseline, there are datapoints below the zero line.  I need to calculate
>area under the curve for the parts of the graph above zero, and then
>subtract area under the curve for the parts below zero.  Does this sound
>feasible via SAS, and if so, is there another method besides the
>trapezoidal that I can use?
>
>Thanks so much for your help.
>
>Melanie




---------------------------------------
Dale McLerran
Fred Hutchinson Cancer Research Center
mailto: dmclerra@fhcrc.org
Ph:  (206) 667-2926
Fax: (206) 667-5977
---------------------------------------

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