In a Quasi-Experiment A $\Delta Y$ You Need to Use $\left( \bar { Y } ^ { \text {after } } - \lambda \times \bar { Y } ^ { \text {before } } \right)$

Short Answer

In a quasi-experiment a. quasi differences are used, i.e., instead of $\Delta Y$ you need to use $\left( \bar { Y } ^ { \text {after } } - \lambda \times \bar { Y } ^ { \text {before } } \right)$ , where $0 < \lambda < 1$ .
b. randomness is introduced by variations in individual circumstances that make it appear as if the treatment is randomly assigned.
c. the causal effect has to be estimated through quasi maximum likelihood estimation.
d. the $t$ -statistic is no longer normally distributed in large samples.