# Thread: how to make a confidence interval??

1. ## how to make a confidence interval??

Hello,
The problem is this:
Tax income is a considerable worry for the local authorities as this income is of vital importance to the service level that the local authorities are able to offer the inhabitants. The data set, “Dataset, August 2011” includes the answers from 87 local authorities regarding the tax income (tax) measured in 1000,- kr., whether the local authority is situated in the northern, southern or eastern regions, number of inhabitants of the local authority (population) and finally the number of inhabitants of the local authority with a long education (long_edu).

• Make a 95% confidence interval of the local authorities’ average tax income.

The way I do:

95% CI = sample mean +- 1.96*standard deviation/sqrt(sample size)

= 1000 +- 1.96* ?/sqrt(87)
So the question is this standard deviation should be TAX or population?

2. ## Re: how to make a confidence interval??

It looks like your "sample mean" is incorrect. You cited the 1000, rather than the actual mean of the sample.

Answering your question, the SD should be of whatever you used to calculate the sample mean.

3. ## Re: how to make a confidence interval??

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Should I multiply mean from 87?
Or it is not necessary ?

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