1. Suppose you are doing a study of long Covid; you have a random sample of Portland residents who have tested positive for the disease, and you gather data on them at one, three, and six months post-infection. Unfortunately, a significant number of them move during the study period, removing them from the sample. How might you check to see in what ways and to what extent the unbiasedness of your sample has been compromised?

2.You are interested in the effect of mask mandates in the schools on community spread of Covid. You locate a city that is half in one state and half in another. In the first state there were mask mandates during the time period you are studying; in the second there weren't. Under what circumstances might this be considered a pseudo-experiment?