Very few 11-year-olds smoke to any extent. Some individuals dramatically increase their intake at age 14, but again the vast majority uses no cigarettes (the median at all young ages is zero). By the age 17 assessment the mean CPD increases to around 2 for females and 3 for males. By age 20 and 24 the mean is around 4 for females and 6 for males. As observed in the figure, the standard deviation of CPD increases with the mean, a consequence of the fact that most individuals use zero to one CPD regardless of age. The “dip” at around age 17, visible for males and females, appears to represent the switch from computerized assessment of CPD to in-person interview assessment. Another possibility is that the dip is due to a cohort effect, as the older cohort begins their assessment at age 17, and may have been using cigarettes to a less extent than the younger cohort, resulting in the observed dip. Plotting a running mean only on the younger cohort, excluding the older cohort, yields the same age-17 dip in CPD, and suggests that the decline in CPD is due to the change in assessment format.
