The primary issue is to determine sample size required to meet
the necessary conditions for a valid study.
The cross-over issue is not significant. View the study as a
2-part study where Group-1 is the Control, and Group-2 is the Study.
Then, Group-1A is the Study, and Group-2A is the Control.
Data can be grouped together and in effect increase the total
sample size to the total numbers in the individual wards.
To determine sample size, first determine the Confidence
Interval desired and the Level of Acceptable Error.
For example: 95% CI (corresponds to a Z-Score = 1.96) and 2%
Acceptable Error, and Variance Unknown
The formula:
N = (1.96^2)(Sigma_Squared Assumed = 1/4)/(Error^2)
Review any good stats text on the topic of Sample Size, and you
should see this basic formula and its explanation.
Consider, Cochran's "Sampling Techniques, 3rd Edition" for a
good resource text dealing with this type of problem and others.
Respectfully,
Michael J. Reeves
On 5 Dec 2002 09:51:39 -0800, [EMAIL PROTECTED] (Bernard
Higgins) wrote:
>Hi
>
>I have been approacted by a specialist registrar wishing to conduct a study in which
>a
>treatment is to be given to patients to reduce the incidence of diarrhoea. He plans
>to
>use two wards in one hospital. All new patients attending one ward will be allocated
>to
>the test treatment and all new patients in the other ward will form a control group.
>The
>outcome of interest is incidence of diarrhoea during one month of follow up. He then
>plans to allow for a "washout" period, of up to one month, after which the two wards
>crossover to the other treatment. His question was; how many patients do I need? (He
>didn't think about how he might analyse his results!)
>
>My first reaction was that this is a cluster randomised trial with one cluster per
>arm
>(ignoring the "crossover" for the moment) and in the absence of any background
>information regarding likely incidence in the control group, I was not in a position
>to
>offer much advice.
>
>I do not feel that a crude linear contrast of incidence rates involving the two
>hospital
>wards over the two treatment periods is really appropriate. What do othes feel? Can
>anyone suggest a sensible way forward?
>
>
>regards
>
>Bernie
> -----------------------------------------
> Bernie Higgins, email: [EMAIL PROTECTED]
>
> Department of Mathematics, University of Portsmouth
> Room 18, Floor 1, Buckingham Building
> Lion Terrrace, Portsmouth PO1 3HE
>
> Tel 023 92 846361 Monday & Tueday
> 023 92 286236 Wednesday - Friday
>
>
>
>
>
>.
>.
>=================================================================
>Instructions for joining and leaving this list, remarks about the
>problem of INAPPROPRIATE MESSAGES, and archives are available at:
>. http://jse.stat.ncsu.edu/ .
>=================================================================
(REMOVE "NoSPAM." from REPLY address to reply)
(This email scanned by Norton Anti-Virus and Certified VIRUS FREE!)
Michael J. Reeves, AA, ASc
E-Mail: [EMAIL PROTECTED]
---------------------------------------------------------
I have no SPAM. I don't give a SPAM.
I take no SPAM from anyone. I am NOT in the SPAM business!!!
.
.
=================================================================
Instructions for joining and leaving this list, remarks about the
problem of INAPPROPRIATE MESSAGES, and archives are available at:
. http://jse.stat.ncsu.edu/ .
=================================================================
