A researcher wishes to estimate, with 90% confidence, the population proportion of adults who say chocolate is their favorite ice cream flavor. Her estimate must be accurate within 11%of the population 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 20% of the respondents said their favorite flavor of ice cream is chocolate.

(c) Compare the results from parts (a) and (b).