those with the weakest are at the bottom.

© John Wiley & Sons, Inc.

FIGURE 7-2: Levels of evidence in study designs.

Starting at the base: Expert opinion

At the base of the study design evidence pyramid shown Figure 7-2 is a descriptive study design:

expert opinion. When the condition of dementia was first identified, few clinicians had seen patients

with dementia. These clinicians served as experts who would write about the dementia patients they

treated and share their experiences at medical conferences. This is what is meant by expert opinion,

and while it is helpful when conditions are first identified, expert opinion is considered a very weak

descriptive study design.

Making the case with case studies

Also at the base of Figure 7-2 are case studies and case series. To develop an understanding of

dementia when it was first identified, clinicians treating patients needed to study them. They would

write up case studies or case reports on individual patients describing their symptoms and providing

the best descriptive evidence as possible. If the clinician was able to identify more than one patient,

they could write about a series of patients, which is known as a case series. While case studies and

case series are helpful for researchers when a condition is first identified, they are considered as

providing very weak evidence to use for causal inference.

Making statements about the population

Ecologic studies (also called correlational studies) and cross-sectional studies appear in the

next level up from expert opinion and case studies and case series. Though ecologic studies are

still descriptive designs that provide weak evidence, they have the advantage of having

potentially very large samples.

In ecologic studies, the experimental units are often entire populations (such of a region or country).