In: Statistics and Probability
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)
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