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Evaluating the placebo response in clinical trials of moderate-severe psoriasis affords the opportunity to assess the effect on psoriasis of temporal changes in ambient solar ultraviolet radiation exposure (UVR), without confounding from use of systemic, ultraviolet, or potent topical therapies, which are prohibited during trial conduct. Anonymized placebo-treated subject data, including dates and PASI scores for baseline and Week 16 visits and approximate investigator location [i.e., first 3 zip code digits, which served as a proxy for subject location], pooled from the placebo-controlled double blinded periods (Weeks 0-16) of 3 moderate-severe psoriasis clinical trials (NCT00237887, NCT02684370, NCT02684357), were accessed through the Vivli data platform. Investigator location was manually geocoded and linked to estimated mean daily ambient erythemally weighted UVR for the months corresponding to baseline and Week 16, using data from the Ozone Monitoring Instrument on board the NASA EOS Aura spacecraft, and the German climate center. Simple linear regression of the percentage change in PASI score from baseline to Week 16 on percentage change in UVR between the months of the two visits was performed using the R statistics package. Simple linear regression of percentage PASI change from 538 placebo-treated subjects with non-missing observations yielded a beta coefficient for percentage change in UVR of -0.009 (standard error ¼ 0.008), with a p-value of 0.243 and adjusted R-squared value of 0.0007. Lack of statistical significance was observed across multiple regression analyses adjusting for baseline covariates and for interaction terms, and for mean difference analysis testing whether PASI50 responders versus nonresponders had a significantly greater increase in UVR. Multiple analyses of this dataset failed to demonstrate evidence that temporal variations in ambient solar UVR are associated with clinically relevant or statistically significant variations in psoriasis severity.