Question

For each of the following examples, imagine that some researcher has found the reported pattern of covariation between X and Y. Can you think of a variable Z that might make the relationship between X and Y spurious?

(a) The more firefighters (X) that go to a house fire, the greater property dam- age that occurs (Y).

(b) The more money spent by an incumbent member of Congress’s campaign (X), the lower their percentage of vote (Y).

(c) Increased consumption of coffee (X) reduces the risk of depression among women (Y).

(d) The higher the salaries of Presbyterian ministers (X), the higher the price of rum in Havana, Cuba (Y).

Answer 1

(a)

The intensity of house fire (Z) may affect both number of firefighters (X) that go to a house fire and the amount of property dam- age that occurs (Y).

(b)

Low confidence level of winning the election (Z) will led to more money spent by an incumbent member of Congress’s campaign (X) and also the lower their percentage of vote (Y).

(c)

Age (Z). Young women have high consumption of coffee (X) and have lower risk of depression (Y). Old age may have low consumption of coffee (X) and have high risk of depression (Y)

(d)

Increased Inflation in Havana, Cuba (Z) may led to high salaries of Presbyterian ministers (X) and also the higher the price of rum in Havana, Cuba (Y).