Hello,

I have some older code which used the sampsi command (https://www.stata.com/help.cgi?sampsi) and I would like to know what the method(method) option refers to, specifically, what is the difference between "post" "change"? The seem to give very different results. See below for example:

You can see that the estimated power for post and change is very different results.

(as an aside, I know what the newer stata command is power, how might I implement this power calculation using the power command?)

I have some older code which used the sampsi command (https://www.stata.com/help.cgi?sampsi) and I would like to know what the method(method) option refers to, specifically, what is the difference between "post" "change"? The seem to give very different results. See below for example:

You can see that the estimated power for post and change is very different results.

(as an aside, I know what the newer stata command is power, how might I implement this power calculation using the power command?)

Code:

sampsi 351 363, sd1(405) sd2(405) n1(2000) n2(4000) pre(8) post(8) onesided r1 > (.3) Estimated power for two samples with repeated measures Assumptions: alpha = 0.0500 (one-sided) m1 = 351 m2 = 363 sd1 = 405 sd2 = 405 sample size n1 = 2000 n2 = 4000 n2/n1 = 2.00 number of follow-up measurements = 8 correlation between follow-up measurements = 0.300 number of baseline measurements = 8 correlation between baseline measurements = 0.300 correlation between baseline & follow-up = 0.300 Method: POST relative efficiency = 2.581 adjustment to sd = 0.622 adjusted sd1 = 252.110 adjusted sd2 = 252.110 Estimated power: power = 0.537 Method: CHANGE relative efficiency = 5.714 adjustment to sd = 0.418 adjusted sd1 = 169.424 adjusted sd2 = 169.424 Estimated power: power = 0.827 Method: ANCOVA relative efficiency = 6.442 adjustment to sd = 0.394 adjusted sd1 = 159.573 adjusted sd2 = 159.573 Estimated power: power = 0.865

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