```Date: Mon, 3 May 2004 13:19:43 +0200 Reply-To: Spousta Jan Sender: "SPSSX(r) Discussion" From: Spousta Jan Subject: Re: syntax to compute a common standard deviation - is there a pr oblem with the syntax I have been using Comments: To: Bob Green Content-Type: text/plain; charset="iso-8859-2" Hi Bob, I suspect, that the calculator gives not a 'pure' standard deviation, but sd times a quantile of a Student's distribution - 0,63 * 1,96 is very close to the calculator's result. One more advice: It is better to write formulae in one row, if they are not too complicated: compute s = sqrt(((n1-1)*s1**2 + (n2-1)*s2**2)/(n1+n2-2)). But your method also leads to correct results, I think, although it is not so easy to check it. HTH Jan -----Original Message----- From: Bob Green [mailto:bgreen@dyson.brisnet.org.au] Sent: Monday, May 03, 2004 1:02 PM To: SPSSX-L@LISTSERV.UGA.EDU Subject: syntax to compute a common standard deviation - is there a problem with the syntax I have been using I am wanting to compute a common standard deviation to perform a power analysis of a t-test. I found the following formula: s = sqrt[((n1-1)s1^2 + (n2-1)s2^2)/(n1+n2-2)] I wrote the no-doubt inefficient syntax (below) to perform this operation. Sample values are: n1 =45 s1 =.6451059 (mean = 1.644444) n2 =47 s2 = .6135986 (mean 2 =1.595745) When I used the syntax below I arrived at a value of .63 , whereas when I entered this data into a power calculator the value was 1.2408 ( the correct mean difference was reported .0487) Any advice is appreciated, Bob COMPUTE n1minus = n1-1. EXECUTE . COMPUTE n2minus = n2-1. EXECUTE . compute sd1sq = s1*s1. EXECUTE . compute sd2sq = s2*s2. EXECUTE . compute nom = n1minus*sd1sq + n2minus*sd2sq . EXECUTE. compute denom = (n1 +n2) -2. EXECUTE. COMPUTE nomdenom = nom/denom. EXECUTE. COMPUTE comstd = SQRT (nomdenom). EXECUTE. ```

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