Which marketing metric is specifically used by leading mathematics textbook publishers to track the effectiveness of their digital learning platforms?

Marketing mathematics is a growing field where statistical analysis meets consumer psychology. Educational technology companies are constantly developing new mathematical tools and approaches to market their products effectively. This trivia question focuses on how mathematics education product companies use specific metrics to track and improve their market performance. Test your knowledge of the intersection between mathematics education products and marketing analytics!

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Knowledge Retention Index (KRI) - measuring how well students retain mathematical concepts after using the platform
Visual Engagement Score (VES) - measuring how long users look at graphical elements in the interface
Problem Completion Ratio (PCR) - measuring only the percentage of problems students attempt vs. complete
Teacher Adoption Quotient (TAQ) - measuring only how quickly teachers implement new features