Cecilia wrote:
I would like to determine the sample size for a RCT. Control and
intervention
groups will have the same size.
The intention of the intervention is to increase a certain preventive
measure
among patients.
We will measure prevalence of the preventive measure at baseline for
both groups
and expect to find that 20% of all the patients (control and
intervention) adopt
the measure before the experiment.
We would like to be able to detect a change from 20% to 40% in the
intervention
group, while we expect that the control group will change their adoption
rates
from 20% to 25%.
My first plan was to simulate some samples, starting with some
hypothetical
sample size, then run a logistic model multiple times, and calculate
power as a
proportion of times when the expected coeficients were significant. Then
repeat
the simulations for different sample sizes until I could had an idea of
sample
size versus power.
-------------------------------------------------------------------
-search sample size- points, among other things, to the official Stata
command
-sampsi-.
A change from 20% to 40% means an increase of 25% among the 80%
non-adopters
at baseline, and a change from 20% to 25 % means an increase of 6.25%.
The command now is:
. sampsi .25 .0625
Estimated sample size for two-sample comparison of proportions
Test Ho: p1 = p2, where p1 is the proportion in population 1
and p2 is the proportion in population 2
Assumptions:
alpha = 0.0500 (two-sided)
power = 0.9000
p1 = 0.2500
p2 = 0.0625
n2/n1 = 1.00
Estimated required sample sizes:
n1 = 88
n2 = 88
Now, the 88 + 88 are the numbers for 80% non-adopters at baseline, so
you
should add 25% to these figures, i.e. 110 + 110. - if you want
alpha=0.05
and power=0.90.
Hope this helps
Svend
__________________________________________
Svend Juul
Institut for Folkesundhed, Afdeling for Epidemiologi
(Institute of Public Health, Department of Epidemiology)
Vennelyst Boulevard 6
DK-8000 Aarhus C, Denmark
Phone: +45 8942 6090
Home: +45 8693 7796
Email: [email protected]
__________________________________________
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