Consider the following approaches for comparisons. Which seem safe from confounding, and which appear contaminated by confounding? Identify a possible confounding factor, if you think one exists.
a. A comparison of two types of promotional displays using average daily sales in one store with one type of display and another store with a different type.1
b. A comparison of two types of promotional displays using average daily sales in one store with one type of display on Monday and the other display on Friday.2
c. A comparison of two credit-card solicitations that sends the two types at random to potential customers in the same zip code.3
d. A comparison of highway driving accident rates for male and female drivers using national accident rates over the last year.4
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