The maker of a cell phone screen protector would like to estimate the proportion of customers who file a warranty claim. To do so, they select a random sample of 200 customers and determine that the 96% confidence interval for the true proportion of customers who file a warranty claim to be 0.15 to 0.28. Which of the following would decrease the margin of error?

For a proportion confidence interval, the margin of error is the critical value times the standard error. Decreasing the confidence level lowers the critical value, so the margin of error becomes smaller. Thus, decreasing the confidence level is the correct choice; increasing it would instead make the margin of error larger.

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