The average life expectancy for Pakistan’s population as a whole is 73.9 years, but children born in Quetta will have an average of 77 years, and those born in Karachi 71.7 years. If a newlywed couple from Karachi were to begin their family in Quetta, therefore, their children would be expected to live longer than would be the case if the family remained in Karachi.
Which of the following, if true, would most seriously weaken the conclusion drawn in the passage?