Are these independent? - $\chi^2$-test

36 Views Asked by At

The below table gives us the number of books issued from a certain library on the various days of the week.

enter image description here

Test at $5\%$ level whether issuing book is a day dependent (Given $\chi_{5:0.05}=11.07$)

$$$$

We have to use the chi-square test of independence, right?

H0: Issuing book is a day independent.

H1: Issuing book is a day dependent.

The sum of the row is equal to $120+130+110+115+135+110=720$

We have $\chi_{5:0.05}=11.07$.

Which degrees of freedom do we have here?

Then we have to calculated the corresponding p-value, right?

1

There are 1 best solutions below

1
On BEST ANSWER

I'm no expert, so take my answer with a grain of salt, although I did study Chi-squared tests recently, however, I'm not certain it is the "best test", or I have "applied it in the best way", or whatever. But I feel like in statistics these things are arguable depending on your level. Anyway, this is how I would do it based on the OCR A-Level Further Maths Statistics syllabus.

Quoting from Further Maths OCR A Level Statistics:

A constraint is a parameter in the model which is found from the data and then fixed to take the same value in the model. One constraint that is always required is the total frequency: you always want the total expected frequency to be the same as the observed frequency, otherwise you are not comparing the results fairly.

Our "model" is a table of expected frequencies where the numbers in each box are $120$, because that's what you would expect if the days are all independent.

Degrees of freedom = number of bins - number of constraints = $6-1$ (the total number of books) $=5.$

To be clear, there is $1$ constraint: the total number of books.

So from the table of critical values, the critical value is $\chi_{5:0.05}=11.07.$

Comparing the data with the observed frequencies, we get:

$\chi^2_{\text{calc}}=\frac{(120-120)^2}{120} + \frac{(130-120)^2}{120} + \frac{(110-120)^2}{120} + \frac{(115-120)^2}{120} + \frac{(135-120)^2}{120} + \frac{(110-120)^2}{120} = \ldots$

If $\chi^2_{\text{calc}} > 11.07$ then reject $H_0$ as there is significant evidence of dependence. If $\chi^2_{\text{calc}} < 11.07$ then accept $H_0$: there is no significant evidence of dependence.

You then should write a conclusion, putting your result into the context of the question, as you usually do with hypothesis tests.

I'm not sure what $p-$values have to do with this?