What is per protocol analysis?

Researchers evaluated the efficacy of 4% dimeticone lotion for the treatment of head louse infestation. A randomised controlled trial was performed. Control treatment was 0.5% phenothrin liquid, the standard treatment. Treatments were applied twice, seven days apart, with dimeticone lotion for eight hours or overnight and phenothrin liquid for 12 hours or overnight. One of the main outcome measures was cure of infestation after the second application, regardless of whether reinfestation occurred later.1

Participants were 214 young people (aged 4-18 years) and 39 adults with active head louse infestation; 127 were allocated to 4% dimeticone lotion and 125 to 0.5% phenothrin liquid. Per protocol analysis indicated that a greater proportion of patients were cured with phenothrin—84 of 121 (69%) participants were cured with dimeticone versus 90 of 116 (78%) with phenothrin, a difference of −8% (95% confidence interval −19% to 3%). …