Question

use of chi square

Answer 1

wat

Answer 2

You mean a chi-squared distribution?
It's widespread use in almost anything relating to hypothesis testing (e.g. confidence intervals, statistical inference, et al).

Answer 3

It is widely used in inference and hypothesis testing to analyse associations. In categorical factors.

Like is a factor like smoking (maybe in a non-smoking control group, a 1 pack a day group and a 2 pack a day group) associated with some outcome like increases in number of deaths from lung cancer or associated with more aggressive forms of lung cancer? You would use a chi square procedure to construct a confidence interval or run a hypothesis test or whatever to determine whether or not cigarette smoking is associated with bad outcomes.