Question

The Fish and Game Department stocked a lake with fish in the following proportions: 30% catfish, 15% bass, 40% bluegill, and 15% pike. Five years later it sampled the lake to see if the distribution of fish had changed. It found that the 500 fish in the sample were distributed as follows.

Catfish, Bass, Bluegill, pike
112, 95, 210, 83
In the 5-year interval, did the distribution of fish change at the 0.05 level?

1. Find the value of the chi-square statistic for the sample. (Round the expected frequencies to at least three decimal places. Round the test statistic to three decimal places.)
2. Find or estimate the p- value of sample test statistic?

Answer 1

1)
`\chi^2 =((112-150)^2)/(150)+((95-75)^2)/(75)+((210-200)^2)/(200)+((83-75)^2)/(75)=16.313`

2)
`p_v =P(\chi^2_(3)>16.313)=0.000978`

And we got the same decision reject the null hypothesis at 5% of significance.

Explanation:

Previous concepts

The Chi-Square test of independence is used "to determine if there is a significant relationship between two nominal (categorical) variables". And is defined with the following statistic:

`\chi^2 =\sum_(i=1)^n ((O-E)^2)/(E)`

Where O rpresent the observed values and E the expected values.

State the null and alternative hypothesis

Null hypothesis: The distribution is 30% catfish, 15% bass, 40% bluegill, and 15% pike

Alternative hypothesis: The distribution is NOT 30% catfish, 15% bass, 40% bluegill, and 15% pike

The observed values are given by the table given:

Catfish =112, BAss = 95, Bluegill=210, Pike=83

Calculate the expected values

In order to calculate the expected values we can use the following formula for each cell of the table

`E = \% Grand total`

`E_(Catfish)=500*0.3=150`

`E_(Bass)=500*0.15=75`

`E_(Bluegill)=500*0.4=200`

`E_(Pike)=500*0.15=75`

Part 1: Calculate the statistic

`\chi^2 =((112-150)^2)/(150)+((95-75)^2)/(75)+((210-200)^2)/(200)+((83-75)^2)/(75)=16.313`

`\chi^2 =16.313`

Calculate the critical value

First we need to calculate the degrees of freedom given by:

`df= (categories-1)=(4-1)= 3`

Since the confidence provided is 95% the significance would be
`\alpha=1-0.95=0.05` and we can find the critical value with the following excel code: "=CHISQ.INV(0.95,3)", and our critical value would be
`\chi^2_(crit)=7.815`

We can calculate also the p value:

`p_v =P(\chi^2_(3)>16.313)=0.000978`

And we got the same decision reject the null hypothesis at 5% of significance.