In most countries including the United States, men have higher incomes than women. Does this hold when comparing middle class heads of households (HHSEX: 1 = male, 2 = female)?

a. Is there sufficient evidence that male heads of households have higher incomes than do female heads of households (INCOME)?

b. Estimate the difference between male and female heads of middle class households.

c. What are the required conditions? Are they satisfied?