# Sample size - chi-squared vs. normal

#### pangel

##### New Member
Hi,

I apologise if this is the wrong thread.

I need to calculate sample sizes, but I'm curious as to why the commonly used Krejcie-Morgan tables use a chi-squared formula rather than a student-t or a normal distribution. I see quite a few other papers that use the latter distributions instead. Is this related to an assumption about the population following a normal distribution?

Thank you!

#### BGM

##### TS Contributor
After searching a while, seems that the formula is coming from constructing the confidence interval for proportion in a small sample, using hypergeometric distribution to model the observations, and using normal approximations for the hypergeometric distribution as well. Thats why you come up the square of normal quantile (which is the chi-square quantile).

#### pangel

##### New Member
Thank you - could you link me to some explanations/steps on this?