Date:         Fri, 24 Sep 2010 15:57:46 -0500
Reply-To:     Robin R High <rhigh@UNMC.EDU>
Sender:       "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From:         Robin R High <rhigh@UNMC.EDU>
Subject:      Re: Sample size calculation for non-inferiority test of
              differences between means
Comments: To: Paul Miller <pjmiller_57@YAHOO.COM>
In-Reply-To:  <377904.56577.qm@web57001.mail.re3.yahoo.com>
Content-Type: text/plain; charset="US-ASCII"

Paul,

I wouldn't quibble too much about the precise standard deviation, since
power is an approximation based on population parameters, of which the
sample estimate is often reasonable for a ballpark figure (depending on
how big is your ballpark), so you are on the right track (though I wonder
if survival analysis would be better for this example than a t-test?)


DATA sss;
n1=120;
n2=125;
diff=18-12;
p=.0001/2;  *one sided pvalue;
t=abs(TINV(p,n1+n2-2));
vr = (  ((diff)/t)/ (sqrt(1/120 + 1/125)))**2;
sd = sqrt(vr);
run;

proc print; run;

Obs     n1     n2    diff       p         t          vr         sd

 1     120    125      6     .00005    3.95615    140.826    11.8670


in my ballpark, 12 is close enough, or even 10 12 14 to see how sensitive
the change is to the std dev.

proc power;
twosamplemeans test = diff
dist = normal
groupmeans = 12 | 12
stddev = 10 12 14
nulldiff = 1 3
sides = L
alpha = 0.05
ntotal = .
power = .80
;

run;

Robin High
UNMC






From:
Paul Miller <pjmiller_57@YAHOO.COM>
To:
SAS-L@LISTSERV.UGA.EDU
Date:
09/24/2010 01:22 PM
Subject:
Sample size calculation for non-inferiority test of differences   between
means
Sent by:
"SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>



Hello Everyone,

Given limited information, I would like to conduct some rough sample size
calculations for a non-inferiority test of differences between means.

These are the results I have from a previous study:

N for Treatment = 120
N for Control = 125
Observed difference in recovery time = 12 days vs. 18 days
P-value = 0.0001

I need to do a sample size calculation comparing the previous treatment to
a new one. The test here will be a non-inferiority test of differences
between means. Two sample size calculations need to be done. One will look
at a difference of 1 day and one will look at a difference of 3 days.

The code I've worked out so far for doing this is pasted below and an
explanation of what I'm trying to do follows.

**** Figure out what polled SD was based on abstract;
**** Need this for power calculations;

**** Determine t value that generates p = .0001 on N - 2 df;

data _null_;
format x pvalue6.4;
x = (1-probt(3.9,243))*2;
put x;
run;

**** Determine what pooled variance yields the observed t value;

data _null_;
x = (18-12) / sqrt(145 * (1/120 + 1/125));
put x;
run;

**** Take square root of pooled variance to get common SD;

data _null_;
x = sqrt(145);
put x;
run;

**** Calculate sample size needed to demonstrate non-inferiority;

**** Difference of 1 day;

proc power;
twosamplemeans test = diff
groupmeans = 12 | 12
stddev = 12.041594579
nulldiff = 1
sides = L
alpha = 0.05
ntotal = .
power = .80
dist = normal;
run;

**** Difference of 3 days;

proc power;
twosamplemeans test = diff
groupmeans = 12 | 12
stddev = 12.041594579
nulldiff = 3
sides = L
alpha = 0.05
ntotal = .
power = .80
dist = normal;
run;

So the idea is to determine what t-value would have generated p = 0.0001
on 243 (N-2) df, to determine what pooled variance would have generated
that t-value by plugging values into the formula for t, and then to take
the square root of that value as an estimate of the common standard
deviation for my power analyses.

So I guess I have 2 questions:

1. Is what I'm doing essentially correct including the actual power
analyses?

2. Is there a better/less ridiculous way of doing this?

Thanks,

Paul