CHAPTER 5 Conducting Clinical Research 65

participate for practical reasons (such as safety, or risk of privacy breach).

A reasonable exclusion criterion for a study of a lipid-lowering treatment

would be, “Participants who are not willing to change their medication

during the study are not eligible to participate.”»

» ^{Withdrawal criteria apply to the follow-up portion of the study. They}

describe situations that could arise during the study that would put the

participant in a state where participation should no longer take place. One

example would be that the participant is diagnosed with Alzheimer’s disease

during the study and can no longer make decisions on their own. A typical

withdrawal criterion may be, “If the participant no longer has decision-

making capacity, they will be withdrawn.”

Choosing the structure of a clinical trial

Many clinical trials include a comparison of two or more interventions. These

types of clinical trials typically have one of the following structures (or designs),

each of which has pros and cons:»

» ^{Parallel: In this clinical trial design, each participant receives one of the}

interventions, and the groups are compared. Parallel designs are simpler,

quicker, and easier for each participant than crossover designs, but you need

more participants for the statistics to work out. Trials with very long treatment

periods usually have to be parallel.»

» ^{Crossover: In a crossover design, each clinical trial participant receives all}

the interventions in sequence during consecutive treatment periods (called

phases) separated by washout intervals (lasting from several days to several

weeks). Crossover designs can be more efficient because each participant

serves their own control, eliminating inter-participant variability. But you can

use crossover designs only if you’re certain that at the end of each washout

period, the participant will have been restored to the same condition as at the

start of the study. This may be impossible for studies of progressive diseases,

like cancer or emphysema, or for drugs that last a long time in the body and

are hard to wash out, like SSRIs and marijuana.

Using randomization

Randomized controlled trials (RCTs) are the gold standard for clinical research (as

described in Chapters 7 and 20). In an RCT, the participants are randomly ­allocated