```Date: Tue, 16 Jan 2007 09:50:32 -0800 Reply-To: Dale McLerran Sender: "SAS(r) Discussion" From: Dale McLerran Subject: Re: problem of parametric trend test In-Reply-To: <200701160601.l0FN4NCe001236@mailgw.cc.uga.edu> Content-Type: text/plain; charset=iso-8859-1 --- Ankur Arora wrote: > Hi All, > > > I want to perform parametric trend test in sas. > > Can anybody advise how to perform the parametric trend test in sas. > > Thanks in advance. > > Thanks > Ankur > Trend tests are generally performed by specifying an appropriate linear combination vector in a CONTRAST statement. So, that begs the question "What is an appropriate linear combination vector?" For a linear trend test, an appropriate linear combination has a coefficient sum which equals zero. Moreover, the spacing between levels should be constant. So, if you have a categorical variable with three levels, then an appropriate linear combination vector could have coefficients -1, 0, and 1. Notice that the coefficients sum to zero and are spaced at an interval of 1. If you have a categorical variable with four levels, then an appropriate linear combination vector would be -3, -1, 1, 3. Again, the sum of the coefficients is zero. The spacing between coefficients is 2. (You could specify coefficients -1.5, -0.5, 0.5, 1.5 which also sum to zero and which have spacing 1.) I'll let you figure out what the appropriate contrast coefficients would be if you have a categorical variable with 5 or more levels. So, you have determined the appropriate contrast coefficients. What do you do with them? Most of the regression procedures that you are likely to be working with (GLM, GENMOD, MIXED, LOGISTIC, etc.) have a CONTRAST statement where you can construct your linear trend test. The form of the CONTRAST statement is CONTRAST <"Label identifying contrast in output"> ; So, suppose that you have named a variable (effect) A on your CLASS statement and on the right hand side of your MODEL statement. Also, let's suppose that variable A has four levels. Then a linear trend test is obtained as CONTRAST "Linear trend test for A" A -3 -1 1 3; HTH, Dale --------------------------------------- Dale McLerran Fred Hutchinson Cancer Research Center mailto: dmclerra@NO_SPAMfhcrc.org Ph: (206) 667-2926 Fax: (206) 667-5977 --------------------------------------- ____________________________________________________________________________________ Looking for earth-friendly autos? Browse Top Cars by "Green Rating" at Yahoo! Autos' Green Center. http://autos.yahoo.com/green_center/ ```

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