Question
Does anyone have a good guide for power calculations for determining the number of patients needed in order for a study to be significant?
A link or document outlining the basics and necessary data needed for the calculation would be very useful.
All Answers (16)
-
Depends on the study. If you can give a bit more details, it would make it easier to give a good answer. -
It also depend on the prevalence of the disease in the community
-
You can begin trying to use this:
http://www.dssresearch.com/KnowledgeCenter/toolkitcalculators/statisticalpowercalculators.aspx
Just to begin with... -
If you have patients randomized for a treatment (for instance), prevalence in the population will be irrelevant for the power calculation.
-
I would contact the biostats group at M D Anderson. Years ago we had a program they provided that gave you those calculations. Dr Barry Brown was involved, and if he is still there, I'm sure would walk you through it. -
It may sound difficult but i guess it is easy, you need to know the prevalence of the disease, population of the community and you the common formula or go to the website http://www.statisticalsolutions.net/pss_calc.php for further help :) -
Hi Caryn,
I had the same problem. I found the following links really useful:
Muller et al. 1984: http://ukpmc.ac.uk/abstract/MED/6542184/reload=0;jsessionid=YG54kPaNnmNyyjqmUQSr.0
Testing et al. 1998: http://www.ucd.ie/vavctest/kierantest/biomedical/html/alternatives-to-animals/Reducing%20the%20use%20of%20Laboratory%20Animals%20in%20Biomedical%20Research.pdf -
Clear and well written article you find attached I think it wil be helpfull to understand basic problem of power calculation and sample size!... Enjoy reading
-
Sorry! wrong article was attached, here the right one!
-
Sorry! wrong article was attached, here the right one!
-
Error is repeated by the computer I will try again!
-
Error is repeated by the computer I will try again!
-
Error is repeated by the computer or by my self!... Sorry again I will try again, "R4" article is that I would like to attached....
-
For prevalence study use this formula to calculate sample size:
n= Z2 P (1-P)
d2
(it is Z square and d square and not Z x 2 or d x 2)
Where n= sample size
Z= Z statistic for a level of confidence
P= expected prevalence or proportion
(in proportion of one: if 20%, P=0.2)
d= precision (in proportion of one; if 5%, d=0.05)
For 95% level of confidence which is conventional, Z value is 1.96.
For sample size calculation in diseased compared to normal i.e. in gene expression studies in normal and in disease, you can calculate the sample size by taking power of the test at 80% (beta=0.8, alfa=0.05, SD=1.96 and mu(0)=1 expected in normal (eg. while performing relative gene expression in normal which is taken as one) and expected mean in disease mu(1)=2 (at two-fold increase in gene expression the study will be able to document with 95% confidence level). At this parameter, the sample size will be 31 cases for a two sided test. (this is just an example) I hope it will be of help to you...
cheers
Biswa -
Dear Caryn, Did you get the answer of your question or not yet. There are many formulas but they have specification based on Methodology and kind of work. you want to do some work based on prevalence of disease OR community kind of work etc, if i know ur proposal or objective then its easy for me to suggest you appropriate formula to calculate sample size for your project. -
Dear Caryn,
I am attaching here a Excel file which I received from Helmut Schutz. It will provide you assistance in estimation of sample size for a clinical study.
