4. Discussion 
colorectal cancer, radon-induced disease, fecal occult blood
COLORECTAL CANCER
This randomized trial sought to replicate our previous findings of the superiority of the radon app on radon knowledge and behaviors over printed brochures in a broad sample of the population in Grand Forks, ND, a city with one of the highest residential radon levels in the U.S. Our most important findings are that, after adjustment for income, the app markedly outperformed brochures in stimulating requests for radon tests, a finding consistent with our previous findings. However, contrary to expectations, the app was significantly inferior to brochures in promoting individuals to return the tests to the lab. Together, these findings provide actionable information for future efforts to prevent radon-induced disease.Both the radon app and brochures were effective in improving radon knowledge (We found that individuals with the lowest income were 80% less likely to order free tests, a finding consistent with the sizable literature on radon testing, e.g., [After adjustment for income, brochure recipients were significantly less likely to request test kits. However, brochure recipients who requested kits were almost twice as likely as app users to return them to the lab (68% vs. 39%). Although based on relatively small numbers (19/28 vs. 14/36, brochure and app, respectively), this finding was contrary to expectation. The discrepancy between requesting a test and returning the test underscores the fact that the question, “What are the determinants of radon testing rates?”, is a complex one that encompasses several different questions and behaviors. That is, in order to test for radon, individuals first must learn about radon, then acquire a test, and then return the test to the lab. Regarding the first step, learning about radon, the app was equivalent or slightly superior to brochures (as evidenced by the greater increase in radon knowledge in the app group). With regard to the second step, acquiring a test, the app was clearly superior. Conversely, concerning the third step, returning the test to the lab, the app was inferior. How can this be understood? We suggest that the processes involved in test acquisition and test return are different. The superiority of the app in the initial two steps appears to be due to its superiority in engaging users. This is evidenced by the difference in loss to follow-up in the two conditions. In the present and our previous trial, loss to follow-up after randomization was approximately 50% less in the app than in the brochure condition (15 vs. 23% and 19 vs. 26%, app vs brochure, respectively) [Conversely, behaviors relevant to returning a radon test may be influenced by factors other than engagement. Unlike the app, the brochures have an actual physical presence. We speculate that the brochures may have provided a tangible reminder to follow through. It is noteworthy that physical reminders, e.g., postcards, have been shown to increase test return rates in an analogous setting, i.e., the use of at-home, fecal occult blood tests for colorectal cancer [Our study has several limitations. First, 19% of participants were lost to follow-up, which could introduce some selection bias. This rate is comparable to average losses to follow-up in other trials of behavior change (i.e., 18%) [Conversely, our study has several strengths: it was randomized, adequately powered, and compared a novel intervention (the radon app) to usual care (brochures) with a sample comprised largely of homeowners in a high-radon city. Moreover, unlike the majority of studies regarding radon testing, which measure subjective attitudes and/or “readiness” to test, we measured actual test acquisition and test returns using objective measures (bar-coded tests) that are independent of self-report [