From a group of flight attendants at Jet Airlines, a researcher randomly selected 60 employees. The flight attendants were divided into two groups by random assignment of 30 to group A, a traditional in-service program on customer attitudes, and 30 to group B, a new program designed to help flight attendants cope with customer attitudes while on the airplane. The two programs were compared at the end of a six- month period by change scores on a scale designed to measure attitudes toward customers. The independent variable is:
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