c) Describe the DifferenceinDifference (DiD) methodology that Bastian uses to estimate the causal effect of the introduction of the EITC in 1975 on mothers' labour supply? How does he define the treatment group and the control group? Describe the assumption that must hold for the DiD to produce an unbiased estimate of the causal effect ofthe EITC on labour supply? How can one test this assumption