http://spssx-discussion.165.s1.nabble.com/Power-estimated-sample-size-and-conundrum-tp5726815.html
Greetings all....I have a
question/situation that on the surface seems very transparent but I am having
difficulties navigating. So any feedback will be much appreciated.
For a study two years
ago I conducted a power analysis comparing two proportions (p1 = .085 vs. p2 =
.045). Whether I used G*Power, Stata, or Power and Precision they all gave me
sample size estimates ranging from 600 to 640 (contingent if continuity
correction was incorporated) per group for alpha = .05 and power of .80.
However, when I ran the
SPSS macro for z test of proportions for two groups comparing .045 vs. .085, I found that n = 300 per
group was sufficient to obtain significance: z = -1.99, p = .047. I also
confirmed this in Stata using the following syntax: prtesti 300 .045 300 .085.
Hence, can I assume that
the difference (i.e., n = 600 per group per power analysis vs. n = 300 per
group being sufficient to obtain significance) is a function of the desired
power insofar with the conventional power of .80 you are setting the bar high
enough so as to find the optimal nexus of Type I and Type II error?
The conundrum is such
that our study ended up with p1 = .027 vs. p2 =.047 and p = .194 with the recommended sample
size being n = 300 per group given the simulation I ran in SPSS and Stata. Note that post hoc power analysis indicates I
would have needed n = 1496 per group (alpha =.05, power = .80) to obtain significance
for delta of 2%, though when I run .027 vs. .047 in Stata and SPSS with n = 685
per group z = 1.96, p = .05.
Anyway, this has become
an interesting/challenging discussion with PI and reviewers alike. We went with the sample size (n = 300 per
group) since that was sufficient for obtaining significance, but they are indicting we
should have gone with the larger sample size based on the power estimate (i.e.,
n = 600 per group).
Has anyone encountered
such a dilemma and how did you deal with it?
Thank you….Dale
Dale Glaser, Ph.D.
Principal--Glaser Consulting
Lecturer/Adjunct Faculty--SDSU/USD/Alliant
3115 4th Avenue
San Diego, CA 92103
phone: 619-220-0602
fax: 619-220-0412
email:
[hidden email]website: www.glaserconsult.com
=====================
To manage your subscription to SPSSX-L, send a message to
(not to SPSSX-L), with no body text except the
command. To leave the list, send the command
SIGNOFF SPSSX-L
For a list of commands to manage subscriptions, send the command
INFO REFCARD