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57 lines
2.2 KiB
Plaintext
57 lines
2.2 KiB
Plaintext
7 months ago
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program define xsampsi
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*version 1.0 Jan Brogger, Jan 31st 2002
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*This program will compute
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*sample size according to senn, 1991, p. 217
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version 7.0
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syntax , alpha(real) beta(real) delta(real) stddev(real) [ n(numlist) ]
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di as text "Sample size for cross-over trial (AB/BA)"
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di "according to Senn 1991: 'Cross-over trials in clinical research'"
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di "Parameters below expressed in terms of the basic estimator, the treatment contrast." _n _dup(15) "-" _n
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di as text "Alpha level" _col(15) " = " as inp %4.3f `alpha' _col(25) as text "Power" _col(35) " = " as inp %4.3f (1-`beta')
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local zalpha=invnorm(1-(`alpha' / 2))
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local zbeta=invnorm(1-`beta')
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di as text "Zalpha " _col(15) " = " as res %4.3f `zalpha' _col(25) as text "Zbeta " _col(35) " = " as res %4.3f `zbeta'
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di _n as text "Detectable effect size" _col(35) " = " as inp %4.3f `delta'
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di as text "Std.dev. of basic estimator" _col(35) " = " as inp %4.3f `stddev'
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local napprox = ((`zalpha'+`zbeta')^2)*((`stddev'/`delta')^2)
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local napprox=int(`napprox')+1
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di _n as text "Approximate N (normal) " _col(35) " = " as res %4.0f `napprox'
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local tdegfree=`napprox'-2
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local tcritical=invttail(`tdegfree',`alpha'/2)
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di as text "Critical value of t(df=" as res `tdegfree' as text "," as res %4.3f `alpha'/2 as text ")" _col(35) " = " as res %4.3f `tcritical'
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local noncent = sqrt(`napprox')*(`delta'/`stddev')
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di as text "Noncentrality parameter of t" _col(35) " = " as res %4.3f `noncent'
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cap nctprob `tcritical' `noncent' `tdegfree'
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if _rc~=0 {
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di as err "Need to install package 'nct' to compute non-central t distribution"
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error 999
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}
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local power = 1-`r(p)'
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di as text "Power given approximate N above " _col(35) " = " as res %4.3f `power'
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if "`n'"=="" {
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di as text _n "(specify other sample sizes in paramter " as inp "n(numlist)" as text " to get power for other sample sizes)"
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}
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else {
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di _n as text "Sample size" _col(15) "Power"
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foreach sampsi of numlist `n' { local tdegfree=`sampsi'-2
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local tcritical=invttail(`tdegfree',`alpha'/2)
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local noncent = sqrt(`sampsi')*(`delta'/`stddev')
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qui nctprob `tcritical' `noncent' `tdegfree'
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local power = 1-`r(p)'
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di as res %4.0f `sampsi' _col(15) %4.3f `power'
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}
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}
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end
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