Current difficulties in achieving intersubjective agreement in linguistics require attention to principles of methodology which consider sources of error and ways to eliminate them. The methodological assumptions and practices of various branches of linguistics are considered from the standpoint of the types of data gathered: texts, elicitations, intuitions and observations. Observations of the vernacular provide the most systematic basis for linguistic theory, but have been the most difficult kinds of data for linguists to obtain; techniques for solving the problems encountered are outlined. Intersubjective agreement is best reached by convergence of several kinds of data with complementary sources of error.