CHALLENGE ACTIVITY 7.2.3: Excel: Confidence intervals for a proportion. Click this link to download the spreadsheet for use in this activity. Critical values for quick reference during this activity. Confidence level| Critical value 0.90 z* = 1.645 0.95 z* = 1.960 0.99 z* = 2.576 Jump to level 1 The manager of a bookstore with a coffee shop wants to know the proportion of customers that come into the store because of the coffee shop. A random sample of 75 customers was polled, Use Sheet 2 of the Excel file linked above to calculate p and the 95% confidence interval. p = Ex: 1.234 Upper bound for 95% confidence interval = Ex: 1.234 Lower bound for 95% confidence interval = The manager can say with 95% confidence that the true population proportion of the customers that come into the store because of the coffee shop is in the interval [ Ex: 1.234 O 5 P