Q. Find the critical value zC necessary to form a confidence interval at the level of confidence shown below.c=0.80
Understand concept critical value: Understand the concept of a critical value in the context of a confidence interval. The critical value zC is the z-score that corresponds to the desired level of confidence. It is the value that captures the central area under the standard normal distribution curve. For a confidence level of 0.80, we want to find the z-score that leaves 10% in each tail (since 100%−80%=20%, and this is split between the two tails of the distribution).
Use distribution table: Use a standard normal distribution table or calculator to find the z-score that corresponds to an area of 0.90 to the left of it.Since we want to leave 10% in the upper tail, we look for the z-score that has 90% of the distribution to its left (0.80 for the middle plus 0.10 for the lower tail).
Find z-score: Find the z-score from the standard normal distribution table or calculator.Using a z-table or calculator, we find that the z-score that corresponds to an area of 0.90 to the left is approximately 1.28.