Negative campaigning seldom works effectively

Unions in the United States _____

a. operate more efficiently than they do in other countries b. can provide better side benefits than they do in other countries c. are able to negotiate better wages than they can in other countries d. have less power with employers than they do in other advanced countries

A study found higher rates of crime in cities with an international airport, compared to cities without an international airport. But the study failed to take account of the size of the city (population), and bigger cities tend both to have more crime and to have international airports. The findings of this study suffer from

a. Measurement bias b. Omitted variable bias c. Sample selection bias d. All of the above