Question: Researchers conducted an RCT to test the effectiveness of a mobile app designed to promote cognitive engagement and reduce the incidence of Mild Cognitive Impairment (MCI) among older adults. At the start of the study, 2,500 adults over the age of 65 were enrolled in the study and randomized to either receive the mobile app intervention ("experimental mobile app") or a placebo mobile app ("placebo"). 1,250 participants were randomized to each group and followed for 24 months. The researchers were able to track how often the participants used their apps, and they decided to only analyze data from those who consistently used the app for at least 20 months. By the end of the study period, 738 people randomized to the experimental mobile app were frequent app users and 48 of these people were diagnosed with MCI; of the infrequent app users in this arm, 157 were diagnosed with MCI. 1,098 people in the placebo arm were frequent app users and 189 of these people were diagnosed with MCI; of the infrequent app users in this arm, 20 were diagnosed with MCI.

1. What kind of analysis did the researchers conduct (intent-to-treat or per-protocol analysis)?
2. Calculate the ratio of cumulative incidence of MCI comparing 1) those randomized to receive the experimental mobile app who were frequent users to 2) those randomized to receive the placebo app who were frequent users. Provide an interpretation of this ratio. What does this value suggest about the effectiveness of the intervention?
3. How would the ratio measure from b) differ if an intent-to-treat analysis were used? Support your answer with calculations. What do these values suggest about the effectiveness of the intervention?
4. Based on your answers to b) and c) above, what would you conclude about the effectiveness of the mobile app in reducing the cumulative incidence of MCI? Briefly explain.
5. The researchers claim the results of their study have larger implications for all mobile app users, regardless of age. Do you agree? Why or why not?

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