Question 4 (10 marks) A researcher has collected a sample of 25 respondents and the mean was calculated as 500. The sample was randomly drawn from a normal population whose standard deviation is 15. a) Estimate the population mean with 99% confidence (4 marks) b) Repeat the part (a) changing the population standard deviation to 30. (2 marks) c) Repeat the part (a) changing the population standard deviation to 60. (2 marks) d) Describe what happens to the confidence interval estimate when the standard deviation is increased. (2 marks)