what were the types of research they conducted?
Formative research and summative evaluations
Sesame Street research
CTW conducted research in two ways: in-house formative research that informed and improved production, and independent summative evaluations conducted by the Educational Testing Service (ETS) during the show's first two seasons to measure the program's educational effectiveness. 
what were the producers of the show sesame street researching about? They were researching about the show's impact on its young viewers' learning.
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Sesame Street research [SEP] Introduction
CTW conducted research in two ways: in-house formative research that informed and improved production, and independent summative evaluations conducted by the Educational Testing Service (ETS) during the show's first two seasons to measure the program's educational effectiveness. CTW researchers invented tools to measure young viewers' attention to the program. Based on these findings, the researchers compiled a body of data and the producers changed the show accordingly. The formative research on "Sesame Street" was the first time children's television viewing was studied scientifically. Summative research conducted over the years, including two landmark evaluations in 1970 and 1971, demonstrated that viewing the program had positive effects on young viewers' learning, school readiness, and social skills.