explicitly defining the criteria a potential participant has to meet to be eligible to be enrolled and

maintained in the study as a participant.

Inclusion criteria are used during the screening process to identify potential participants who are

members of the population about whom you want to draw conclusions. A reasonable inclusion

criterion for a study of a lipid-lowering intervention would be, “Participant must have a

documented diagnosis of hyperlipidemia, defined as total cholesterol

and

at screening.”

Exclusion criteria are used to identify potential participants who do not fall in the population

being studied. They are also used to rule out participation by individuals who are otherwise in the

population being studied but should not 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withdrawn.”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 by intervention into treatment

groups (in a parallel trial) or into treatment-sequence groups (in a crossover design). Randomization

provides several advantages: