For each of the following situations, determine the sign (and, if possible, comment on the likely size) of the expected bias introduced by omitting a variable:
a. In an equation for the demand for peanut butter, the impact on the coefficient of disposable income of omitting the price of peanut butter variable. (Hint: Start by hypothesizing signs.)
b. In an earnings equation for workers, the impact on the coefficient of experience of omitting the variable for age.
c. In a production function for airplanes, the impact on the coefficient of labor of omitting the capital variable.
d. In an equation for daily attendance at outdoor concerts, the impact on the coefficient of the weekend dummy variable (1 = weekend) of omitting a variable that measures the probability of precipitation
at concert time.