What is a factorial study design?

Researchers investigated whether inclusion of glutamine or selenium in a standard isonitrogenous, isocaloric preparation of parenteral nutrition affected the occurrence of new infections in critically ill patients. A randomised double blind placebo controlled trial was conducted, using a full factorial study design. There were two factors—treatment with glutamine (20.2 g/day) or glutamine placebo and treatment with selenium (500 μg/day) or selenium placebo, each for up to seven days. Participants were 502 adults in intensive care units or high dependency units who had gastrointestinal failure and required parenteral nutrition.1

The outcome measure was a new clinically suspected infection within the first 14 days of treatment. The trial found no evidence of an interaction between glutamine and selenium in treatment effect (P=0.96). Selenium supplementation had no significant effect on the development of new infections (odds ratio 0.81, 95% confidence interval 0.57 to 1.15). Glutamine also had no significant effect on the development of new infections (odds ratio 1.07, 0.75 to 1.53). It was concluded that the supplementation of parenteral nutrition with glutamine or selenium had no effect on the development of new infections in critically ill patients.

Which of the following statements, if any, are true?

• a) The trial used the double dummy method

• b) There were four treatment groups

• c) If an interaction existed between glutamine and selenium, the effects of glutamine would have differed between the selenium and selenium placebo treatment groups

• d) The factorial design of the trial required a smaller sample size than investigating the effectiveness of glutamine and selenium in separate trials