A researcher wishes to estimate, with 99% confidence, the population proportion of adults who think the president of their country can control the price of gasoline. Her estimate must be accurate within 3% of the true proportion. a) No preliminary estimate is available. Find the minimum sample size needed. b) Find the minimum sample size needed, using a prior study that found that 26% of the respondents said they think their president can control the price of gasoline. c) Compare the results from parts (a) and (b).

a)  

And rounded up we have that n=1849

b)  

And rounded up we have that n=1423

c) For this case we can see that if we have a prior estimate the minimum sample size required for the margin of error desired would be less as we can see in part b we reduce the sample size compared to the part a

Step-by-step explanation:

Part a

The critical value for a confidence level of 99% is for this case

The margin of error for the proportion interval is given by this formula:  

   (a)  

And on this case we have that and we are interested in order to find the value of n, if we solve n from equation (a) we got:  

  (b)  

Since we don't know any estimate for the true proportion we can use as a godd estimator. And replacing into equation (b) the values from part a we got:

And rounded up we have that n=1849

Part b

For this case we have a prior estimate and replacing we got:

And rounded up we have that n=1423

Part c

For this case we can see that if we have a prior estimate the minimum sample size required for the margin of error desired would be less as we can see in part b we reduce the sample size compared to the part a

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step-by-step explanation: