# Thread: Confidence interval/ Z value

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## Confidence interval/ Z value

Pls anyone clarify what CI really means and how would it be calculate using Z value. (with example pls)

2. ## Re: Confidence interval/ Z value

Originally Posted by nlamichhane
Pls anyone clarify what CI really means and how would it be calculate using Z value. (with example pls)
This should clear it up:

1. Which is wider, a 95% or a 99% confidence interval?

99% is wider

2. When you construct a 95% confidence interval, what are you 95% confident about?

You are 95% confident that the interval contains the parameter.

3. When computing a confidence interval, when do you use t and when do you use z?

You use t when the standard error is estimated and z when it is known. One exception is for a confidence interval for a proportion when z is used even though the standard error is estimated.

4. Assume a researcher found that the correlation between a test he or she developed and job performance was .5 in a study of 25 employees. If correlations under .30 are considered unacceptable, would you have any reservations about using this test to screen job applicants?

You should have reservations since the lower limit of the 95% confidence interval is 0.133, you cannot be confident that the true correlation is at least 0.30.

5. What is the effect of sample size on the width of a confidence interval?

The larger the sample size, the smaller the width of the confidence interval.

6. How might an experimenter demonstrate that a drug has a negligible effect?

By showing that all values in the confidence interval are negligble.

7. When is ******'s z' used? Why is it necessary to use ******'s z'?

It is used to construct a confidence interval on Peason's correlation. It is used because the sampling distribution of r is not normal.

8. What assumptions are made in the construction of a confidence interval on µ?

Normality of the parent population and that each observation is sampled randomly and independently

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very good explanations.

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