Hello,
I need to calculate a minimum sample size for the chi square test.
I have two groups, sick and healthy patients, and I have a new test that should detect the sick patients. The machine tell "sick" or "healthy". I was asked to check if the results of the test (E) are independent to the status (D). This is a simple 2X2 table. What is the required sample size for the appropriate chi square test ?
under H0 my expected frequencies should reflect independence.
so you are saying that I should put in every cell in the table a size of 5 patients, so I need 10 sick and 10 healthy ?
Essentially, what you need is an expected frequency of at least 5 in each of the 4 cells of a 2x2 chi-square design to justify the use of an (asymptotic) approximation of a chi-square statistic under a true null hypothesiss that the two variables are independent...Let your software package compute the expected frequencies.
Hi!
May be Dragan was warning you about the fact that in order for chi-square test to be reliable none of the expected frequencies should be smaller that 5. I have made some search in the web, and I have realized that this is a rather controversial topic....but this is another story...
As for your question, provided that I do not know the size of your sample, you could use Fisher test instead, since you have a 2x2 table. For further info, you could jump to this link.
Hope this helps,
Regards
Gm
Outside of a dog, a book is man's best friend. Inside of a dog it's too dark to read -Groucho Marx-
thank you to both of you !
ginamarco said: "I do not know the size of your sample". Well, this is the problem, I do not have one, I need to decide how many to sample....
So you'll have to do some sample size calculations? Maybe a software like
G*power http://www.psycho.uni-duesseldorf.de...n/aap/gpower3/
or similar sample size calculation software can help you.
Kind regards
K.
thanks, I found another software (PS), and now I understand the rationale behind the calculations.
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