Question

In: Statistics and Probability

The National Weight Control Registry tries to mine secrets of success from people who lost at...

The National Weight Control Registry tries to mine secrets of success from people who lost at least 30 pounds and kept it off for at least a year. It reports that out of 3500 registrants, 922 were on a low-carbohydrate diet (less than 90 grams a day).

a. Develop a 99% confidence interval for the proportion of people in the registry on a low-carb diet.

b. Is it possible that this population proportion is 25%?

c. For a follow-up study, how large a sample is needed to estimate the proportion within .5% and with 99% confidence?

(Please make sure you focus on the numbers given above)

Solutions

Expert Solution

a.Develop a 99% confidence interval for the proportion of people in the registry on a low-carb diet

We need to construct the 9% confidence interval for the population proportion. We have been provided with the following information about the number of favorable cases:

Favorable Cases X = 922
Sample Size N = 3500

The sample proportion is computed as follows, based on the sample size N = 3500 and the number of favorable cases X = 922

The critical value for α=0.01 is z_c = 2.576. The corresponding confidence interval is computed as shown below:

b. Is it possible that this population proportion is 25%?

As the interval contains 0.25, hence yes it is possible that this population proportion is 25%

c. For a follow-up study, how large a sample is needed to estimate the proportion within .5% and with 99% confidence?

The critical value for α=0.01 is z_c = 2.576

Margin of error, E = 0.5% = 0.005

n = 9972.3653

n = 9973


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