UK PRECISE 2006Low.

Methods	Prevention of Cancer by Intervention with Selenium Pilot Study (PRECISEp). Randomised, double‐blind, placebo‐controlled trial with parallel group design (four intervention groups).
Participants	Country: United Kingdom. Number of participants randomised: 501, 53% men at age 60 to 74 years, mean age 67 years. Inclusion criteria: volunteers from four general practices. Exclusion criteria: incapable of carrying out light housework or office work, active liver or kidney disease, prior diagnosis of cancer (excluding nonmelanoma skin cancer), diagnosed HIV infection, immunosuppressive therapy, diminished mental capacity, taking > 50 µg/day of selenium supplements in the previous six months (by patient report).
Interventions	Participants were randomly assigned to receive: group 1: placebo (n = 121); group 2: selenium 100 µg (n = 127); group 3: selenium 200 (n = 127); group 4: selenium 300 µg (n = 126); in the form of high‐selenium yeast, Seleno PreciseTM per day for two years.
Outcomes	The primary outcome measures were: mood, quality of life, and plasma selenium level.
Notes	Compliance with randomised treatment was determined by pill count, with participants considered compliant if they took at least 80% of their allocated tablets. Reasons for participant withdrawal were noted. Four hundred fifty three of the 467 participants (97%) who completed six months were compliant according to pill count. Thirty‐four participants (7%) withdrew from treatment within the first six months. There was no significant difference in treatment withdrawals between groups (7, 10, 5, and 12 in the placebo and 100, 200, and 300 µg groups respectively. Trial agents were provided by Pharma Nord, Vejle, Denmark.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Sequence generation was achieved using computer random number generation.
Allocation concealment (selection bias)	Low risk	Allocation was controlled by a central and independent randomisation unit, so that intervention allocations could not have been foreseen in advance of, or during, enrolment.
Blinding (performance bias and detection bias) All outcomes	Low risk	The trial was described as blinded, the parties that were blinded, and the method of blinding was described, so that knowledge of allocation was adequately prevented during the trial.
Incomplete outcome data (attrition bias) All outcomes	Low risk	The numbers and reasons for dropouts and withdrawals in all intervention groups were described.
Selective reporting (reporting bias)	Low risk	Pre‐defined, or clinically relevant and reasonably expected outcomes are reported on.
Other bias	Low risk	The trial appears to be free of other components that could put it at risk of bias.