Some adult studies report fairly large positive correlations between the amount of classroom exposure to the second language and proficiency. In each case, however, it can be argued that the class was the primary source of comprehensible input. Krashen, Zelinski, Jones, and Usprich (1978) tested students in an extension (evening and weekend) program in English as a second language at Queens College in New York, and reported robust correlations between reported years of formal study and performance on a variety of ESL tests.