Basic Statistics

COMMON TYPES OF BIOMEDICAL STUDIES 5

Subjects Study

Time

2obo i 2001

Year

Disease-free

Time

0

1

.5

1

1

3.5 = Sum

Figure 1.2 Status of five adults followed for a 1-year period.

Person 1 enters the year with the disease already known to be present. Persons
2, 4, and 5 neither have the disease before the year starts nor are diagnosed with the
disease during the year. Person 3 is diagnosed with the disease one-half of the way
through the year. In computing the incidence rate only one-half of the year will be
counted for person 3 since he or she is not at risk to get the disease in the second
one-half of the year. The incidence rate would be U3.5. The prevalence at study time
t would be two-fifths.
For rare diseases, it is common to ignore the time period that is lost due to a patient
getting the disease within the study period. If this is done, a 1 replaces the .5 for the
disease-free time period for person 3 and the incidence rate would be one-fourth. In
this case, the incidence rate is the number of new cases of a disease in a given time
period divided by the population at risk of developing the disease.
The timing of events such as when exposure to risk factors occurs or when the
disease is likely to be diagnosed, and the number of persons who have either the
risk factor or the disease, limits the type of study that can be done. It is critical to
have estimates of the timing and the numbers before designing a biomedical or public
health study.

1.3 COMMON TYPES OF BIOMEDICAL STUDIES

In this section a brief introduction to some of the most common types of biomedi-
cal studies is presented. Further discussion of these studies is given in subsequent
chapters. Here, we are concerned with when the subjects are sampled relative to the
course of their disease or treatment and under what circumstances this type of study
is used. The actual sampling methods are discussed in Chapter 2. (By a sample we
simply mean a subset of a selected population that the investigators wish to study.)
In terms of time, we can examine data taken at an “instant in time”; we can look
forward in time or we can look backward in time. The studies that theoretically take
place in an instant of time are usually called surveys or cross-sectional studies. The
types of studies that look forward in time are often called (1) experiments, (2) clinical
trials, (3) field trials, or (4) prospective or panel or follow-up studies. Clinical trials