Gary Jacobson conducts both aggregate and district-level analyses to test the independent effects of each of these variables on challenger quality. Other scholars find Jacobson's explanation incomplete and argue that additional, district-specific variables account for the emergence of high-quality challengers. As scholars have reached some consensus about the relevance of candidate quality in congressional elections, there exists ample methodological variation in the conceptualization and measurement of the variable in the scholarly literature. In their recent study of candidate quality and electoral competition in congressional races from 1872–1944, Jamie Carson and Jason Roberts find that congressional races during this era, despite having higher levels of quality challengers on average, closely mirror the structure of their contemporary counterparts. In the probit model that estimates the probability that a quality challenger will emerge in a US House race, all of the district-level variables increase the probability that a quality challenger would emerge.