Price differs across regions, so price is one logical explanatory variable. Also, since women typically do more laundry than men, and wealthier people have a higher proportion of clothing that requires dry-cleaning, the researcher also plans to use gender and household income to explain household demand for this product. All variables need to correspond to the same unit of observation . I.e. ALL for sales regions, or ALL for households, not a mixture. We would need to do a joint test to answer this question.
Insufficient info is provided to do an F-test.
Every one of these twenty different empirical models has shown that the coefficient b 2 is negative and strongly statistically significantly different from zero. The empirical evidence is extremely robust across studies. The explanatory variable, VITC i is endogenous , in that its value is determined by the health maintenance decisions of the individual employee. To make a policy recommendation, you would prefer to have a controlled experiment, where VITC i values were randomly assigned to different employees, and their resulting SICK i values recorded. If people who take more vitamin C are more careful about their health in lots of ways, they might be expected to have fewer sick days.