A researcher wishes to evaluate the effectiveness of a one-year job skills training program for disadvantaged women. She has surveyed the women as they entered the training program in January 2015, and then one year after the end of the program, in January 2017, obtaining estimates of their average earnings each year. The women's average earnings in January 2015 were $10,000, and they were $15, 000 in January 2017.

a) "A reasonable estimate of the impact of the training program is that it raised trainee earnings by $5,000." Critically evaluate this statement.

b) The researcher consults her annual reports from Statistics Canada, and reads that the average full-year, full-time female worker in Canada had earnings of $35,000 in January 2015, and $38,000 in January 2017. Can she use this information to construct a better estimate of the impact of training? Can you suggest a better comparison group? Explain.