Tangen’s research interests include the design and analysis of clinical trials, including SWOG trials related to the treatment of genitourinary cancers (such as malignancies of the prostate, bladder, kidney and testes). She served as the primary statistical investigator for the national Prostate Cancer Prevention Trial, which determined that the hair-loss drug finasteride reduced the risk of prostate cancer among healthy men age 55 and older by 25 percent, but in a subgroup of participants it increased the chance of developing clinically significant high-grade cancers by 25 percent. Biospecimens from this trial, which involved 18,800 men across the U.S., have also been used to study the impact of androgen metabolism, diet and diet-related factors, insulin resistance and inflammation, among other variables, on prostate cancer risk.