what were the producers of the show sesame street researching about?
They were researching about the show's impact on its young viewers' learning.
Sesame Street research
Unlike earlier children's programming, the show's producers used research and over 1,000 studies and experiments to create the show and test its impact on its young viewers' learning. 
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Sesame Street research [SEP] Introduction
In 1969, the children's television show "Sesame Street" premiered on the National Educational Television network (later succeeded by PBS) in the United States. Unlike earlier children's programming, the show's producers used research and over 1,000 studies and experiments to create the show and test its impact on its young viewers' learning. By the end of the program's first season, the organization founded to oversee "Sesame Street" production, Children's Television Workshop (CTW), had developed what came to be called "the CTW model": a system of planning, production, and evaluation that combined the expertise of researchers and early childhood educators with that of the program's writers, producers, and directors.