A study was performed in Basel, Switzerland, on relationship between the concentration of plasma antioxidant vitamins and cancer risk. Below shows data for plasma vitamin-A concentration for stomach cancer cases and controls.
Stomach-Cancer cases: Mean: 2.65, Standard Error: 0.11, n = 20
Controls: Mean= 2.88, n= 2421
a. If you assume that the mean plasma vitamin-A concentration among controls is known without error, then what procedure can be used to test if the mean concentration is the same for stomach-cancer cases and controls? State its hypothesis.
b. Perform the appropriate test for question (a), and make a decision with a critical value at alpha = 0.05