A 2021 paper by Callaway and Sant’Anna proposed a variant of this technique to avoid a problematic “two-way fixed effect” specification for “staggered” designs. The “Ashenfelter dip” raises problems for this technique if it is unknown whether a shock is permanent or transitory. A 2004 paper by Marianne Bertrand et al. that asks “How Much Should We Trust [this technique]” simulates inconsistent standard errors using “placebo laws.” A key assumption underlying this technique is the presence of parallel trends in the pre-period. (10[1])Card and Krueger used this technique to study the effect of an increase in minimum wages on employment in Pennsylvania. For 10 points, name this causal inference technique that compares the change in an outcome variable over time between treatment and control groups. ■END■