- Either start therapy with ustekinumab within 30 days of baseline or have started therapy with ustekinumab in the 12-week period before enrollment for the treatment of psoriasis; a. the treatment decision must have been taken independently of and prior to a participants inclusion in the study; b. where participants have started therapy with ustekinumab before enrollment, appropriate baseline data including the disease activity score at the start of ustekinumab treatment must be documented