The Centers for Disease Control and Prevention released a report in 1998 that attributed part of the increase in youth smoking in the 1980s to the use of the cartoon character, Joe Camel, in cigarette advertising. Using annual data going back to 1965, the report showed that the number of people under the age of 18 who started smoking increased markedly in 1988, the same year that Joe Camel first appeared in tobacco ads ("Daily Smoking by Teens Has Risen Sharply," 1998).

a. What kind of a quasi-experimental design was used in this study?

b. What potential threats to internal validity are present in this design?

c. From these data, how confident are you that Joe Camel was responsible for increases in youth smoking?