A study was performed to determine whether moderate to heavy use of alcohol during pregnancy increases the risk of low birthweight infants. The investigators retrospectively reviewed the hospital records of 12,000 women. Of these, 10,000 drank no alcohol (technically none or very rarely), and the other 2,000 drank moderate to heavy amounts of alcohol while pregnant. There were 400 low birth weight infants born to mothers who drank no alcohol, and there were 200 low birth weight infants born to mothers who drank moderate to heavy amounts of alcohol.

Calculate the risk difference, i.e., the excess risk of low birth weight per 100 births among infants whose mothers drank alcohol during pregnancy.

Interpret the risk difference