Just one day before a meeting with your manager, he asks you to get a sample of the incomes of the firm's investors. You call as many as possible and end up with a sample of the first 20 investors who answer. The average income among those 20 is $238,000 with a standard deviation of $80,000.

a)Assume that this is a valid, random sample and that incomes are normally distributed. What is the 95% confidence interval for the average investor's salary?

b)Obviously, this is not a random sample. What kind of sampling did the analyst use to collect this sample (please briefly explain)?

c)Name one kind of bias that might be present in this sample and why.