help sampsi dialog: sampsi
sampsi_repmeas
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Title
[R] sampsi -- Sample size and power determination
Syntax
sampsi #1 #2 [, options]
options description
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Main
onesample one-sample test; default is two-sample
sd1(#) standard deviation of sample 1
sd2(#) standard deviation of sample 2
Options
alpha(#) significance level of test; default is alpha(0.05)
power(#) power of test; default is power(0.90)
n1(#) size of sample 1
n2(#) size of sample 2
ratio(#) ratio of sample sizes; default is ratio(1)
pre(#) number of baseline measurements; default is pre(0)
post(#) number of follow-up measurements; default is post(1)
nocontinuity do not use continuity correction for two-sample test
on proportions
r0(#) correlation between baseline measurements; default is
r0()=r1()
r1(#) correlation between follow-up measurements
r01(#) correlation between baseline and follow-up
measurements
onesided one-sided test; default is two-sided
method(method) analysis method where method is post, change, ancova,
or all; default is method(all)
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Menu
sampsi
Statistics > Power and sample size > Tests of means and proportions
sampsi with repeated measures
Statistics > Power and sample size > Tests of means with repeated
measures
Description
sampsi estimates require sample size or power of tests for studies
comparing two groups. sampsi can be used when comparing means or
proportions for simple studies where only one measurement of the outcome
is planned and for comparing mean summary statistics for more complex
studies where repeated measurements of the outcome on each experimental
unit are planned.
If n1(#) or n2(#) is specified, sampsi computes power; otherwise, it
computes sample size. For simple studies, if sd1(#) or sd2(#) is
specified, sampsi assumes a comparison of means; otherwise, it assumes a
comparison of proportions. For repeated measurements, sd1(#), or sd2(#)
must be specified. sampsi is an immediate command; all its arguments are
numbers; see immed.
For simple studies, where only one measurement of the outcome is planned,
sampsi computes sample size or power for four types of tests:
1. Two-sample comparison of means.
The postulated values of the means are #1 and #2.
The postulated standard deviations are sd1() and sd2().
2. One-sample comparison of mean with hypothesized value.
Option onesample must be specified.
The hypothesized value (null hypothesis) is #1.
The postulated mean (alternative hypothesis) is #2.
The postulated standard deviation is sd1().
3. Two-sample comparison of proportions.
The postulated values of the proportions are #1 and #2.
4. One-sample comparison of proportion with hypothesized value.
Option onesample must be specified.
The hypothesized proportion (null hypothesis) is #1.
The postulated proportion (alternative hypothesis) is #2.
Options
+------+
----+ Main +-------------------------------------------------------------
onesample indicates a one-sample test. The default is two-sample.
sd1(#) and sd2(#) are the standard deviations of population 1 and
population 2, respectively. One or both must be specified when doing
a comparison of means. When the onesample option is used, sd1(#) is
the standard deviation of the single sample (it can be abbreviated as
sd(#)). If only one of sd1(#) or sd2(#) is specified, sampsi assumes
that sd1() = sd2(). If neither sd1(#) nor sd2(#) is specified,
sampsi assumes a test of proportions. For repeated measurements,
sd1(#) or sd2(#) must be specified.
+---------+
----+ Options +----------------------------------------------------------
alpha(#) is the significance level of the test. The default is
alpha(0.05) unless set level has been used to reset the default
significance level for confidence intervals. If a set level #-level
command has been issued, the default value is alpha(1-level/100).
See [R] level.
power(#) = 1 - b is the power of the test. The default is power(0.90).
n1(#) and n2(#) are the sizes of sample 1 and sample 2, respectively.
One or both must be specified when computing power. If neither n1(#)
nor n2(#) is specified, sampsi computes sample size. When the
onesample option is used, n1(#) is the size of the single sample (it
can be abbreviated as n(#)). If only one of n1(#) or n2(#) is
specified, the unspecified one is computed using the formula ratio =
n2()/n1().
ratio(#) is the ratio of sample sizes for two-sample tests: ratio() =
n2()/n1(). The default is ratio(1).
pre(#) specifies the number of baseline measurements (prerandomization)
planned in a repeated-measure study. The default is pre(0).
post(#) specifies the number of follow-up measurements
(postrandomization) planned in a repeated-measure study. The default
is post(1).
nocontinuity requests power and sample size calculations without
continuity correction for two-sample test on proportions. If not
specified, the continuity correction is used.
r0(#) specifies the correlation between baseline measurements in a
repeated-measure study. If r0(#) is not specified, sampsi assumes
that r0() = r1().
r1(#) specifies the correlation between follow-up measurements in a
repeated-measure study. For a repeated-measure study, either r1(#)
or r01(#) must be specified. If r1(#) is not specified, sampsi
assumes that r1() = r01().
r01(#) specifies the correlation between baseline and follow-up
measurements in a repeated-measure study. For a repeated-measure
study, either r01(#) or r1(#) must be specified. If r01(#) is not
specified, sampsi assumes that r01() = r1().
onesided indicates a one-sided test. The default is two-sided.
method(post|change|ancova|all) specifies the analysis method to be used
with repeated measures. change and ancova can be used only if
baseline measurements are planned. The default is method(all), which
means to use all three methods.
Examples
1. Two-sample comparison of mean1 to mean2. Compute sample sizes with
n2/n1 = 2:
. sampsi 132.86 127.44, p(0.8) r(2) sd1(15.34) sd2(18.23)
Compute power with n1 = n2, sd1 = sd2, and alpha = 0.01 one-sided:
. sampsi 5.6 6.1, n1(100) sd1(1.5) a(0.01) onesided
2. One-sample comparison of mean to hypothesized value = 180. Compute
sample size:
. sampsi 180 211, sd(46) onesam
One-sample comparison of mean to hypothesized value = 0. Compute
power:
. sampsi 0 -2.5, sd(4) n(25) onesam
3. Two-sample comparison of proportions. Compute sample size with n1 =
n2 (i.e., ratio = 1, the default) and power = 0.9 (the default):
. sampsi 0.25 0.4
Compute power with n1 = 500 and ratio = n2/n1 = 0.5:
. sampsi 0.25 0.4, n1(300) r(0.5)
4. One-sample comparison of proportion to hypothesized value = 0.5:
. sampsi 0.5 0.75, power(0.8) onesample
Compute power:
. sampsi 0.5 0.6, n(200) onesam
5. Repeated measures:
. sampsi 498 485, sd1(20.2) sd2(19.5) method(change) pre(1) post(3)
r1(.7)
Compute power:
. sampsi 498 485, sd1(20.2) sd2(19.5) method(change) pre(1) post(3)
r1(.7) n1(15) n2(15)
Saved results
sampsi saves the following in r():
Scalars
r(N_1) sample size n_1
r(N_2) sample size n_2
r(power) power
r(adj) adjustment to the SE
r(warning) 0 if assumptions are satisfied and 1 otherwise
Also see
Manual: [R] sampsi
Help: [ST] stpower
