# App To Find Chi Square sample size

#### mathprof

##### New Member
I'm looking for a simple app for my students to use when they are doing a project involving Chi Square and need to estimate the sample size. What's available?

##### New Member
Todd Grande makes the following suggestions on sample size:
At least 80% of the contingency cells must have an expected value > 5
No cell can have an expected value < 3
Ideal sample size is greater than the total number of cells multiplied by 5

#### obh

##### Member
Hi Acer,

Chi-square test isn't only goodness of fit.
In the goodness of fit test: expected value > 5.

Maybe the total number of cells multiplied by 5 is good for approximation, but calculating the power based on the required effect size gives accurate results.

##### New Member
Hi Acer,

Chi-square test isn't only goodness of fit.
In the goodness of fit test: expected value > 5.

Maybe the total number of cells multiplied by 5 is good for approximation, but calculating the power based on the required effect size gives accurate results.
Hi obh

Grande was describing chi-square test of independence when he explained the rules. I assume they apply only to that test. Seems goodness-of-fit tests usually do not have enough contingency cells to apply those rules.

#### obh

##### Member
Hi Acer,

The goodness of fit can be used to test for independence
I assume Grande describe correctly the rule of thumb. I learned that all cells must have expected value > 5. but 80% is probably also okay. at least easier to meet...(more than one rule of thumb)

In order to calculate the sample size, you need to determine the test's power (usually 0.8, say the probability of 0.2 not to reject incorrect H0 )
and the power depends on the effect that the test needs to recognize. the smaller the effect the weaker the power and you need to increase the sample size to meet the required power