$1.$ Briefly define the following terms $(1-2$ sentences each$),$ and provide an example from the readings $($such as:

$$an example of an instrumental variable is the dummy for same$-$sex siblings in the study of parental investment in

children$$s education$).$

a$.$ Difference$-$in$-$differences strategy

b$.$ Fuzzy regression discontinuity

c$.$ Common trends assumption

d$.$ Fixed$-$effects regression

e$.$ Attenuation bias