# Thread: Sample Size for Chi Square test

1. ## Sample Size for Chi Square test

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 ?

2. ## Re: Sample Size for Chi Square test

Originally Posted by NN_STAT
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 ?
Basically, just look at your expected frequencies in your 2X2 table...they shouldn't be less than 5.

3. ## Re: Sample Size for 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 ?

4. ## Re: Sample Size for Chi Square test

Originally Posted by NN_STAT
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.

5. ## Re: Sample Size for Chi Square test

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

6. ## Re: Sample Size for Chi Square test

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....

7. ## Re: Sample Size for Chi Square test

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/

Kind regards

K.

8. ## Re: Sample Size for Chi Square test

thanks, I found another software (PS), and now I understand the rationale behind the calculations.

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