hormone replacement component of the WHI Clinical Trial and Observational
Study (http://www.nhlbi.nih.gov/whi/ctos.htm) is another example of an ex-
perimental follow-up study. In this study, women were randomly assigned to
either a treatment or placebo group and then followed for 8 years to determine
the effect of hormone replacement therapy on the prevention of coronary heart
disease and osteoporotic fractures.
Crossover designs are a type of longitudinal study in which subjects re-
ceive more than one experimental treatment and are then followed over time.
Subjects act as their own control group. Researchers manipulate the IV by
randomizing the order in which the treatments are provided. An example of
a crossover design is one in which the researcher is interested in determining
whether relaxation techniques or exercise has a greater effect on reducing
blood pressure. Some subjects would be randomly assigned to receive training
in relaxation techniques first while others would be given instructions about
exercise first. As subjects enter the study, baseline measures of blood pressure
would be obtained. They would then do either the exercise or relaxation pro-
cedures for a set time period and would have their blood pressure measured at
different points in time. Subjects would then be asked to stop the intervention
and resume their normal habits while blood pressures are measured again. At
a later time, subjects would receive instructions about the experimental treat-
ment they had not done initially. Again, subjects would be followed for a set
period of time with their blood pressures being measured. At the end of the
study, the researcher would be able to determine which intervention produced
the greatest reduction in blood pressures. One problem with crossover studies,
however, is the possibility of a carryover effect. Even though subjects are asked
to stop performing the first intervention, sometimes they may continue to use
it during the second phase of the study.
Gathering data at multiple points in time does not necessarily make a study
longitudinal. Usually the term longitudinal is reserved for studies in which
data are gathered over extended periods of time rather than in just a few
hours or days. In some nursing situations, data may be repeatedly gathered
but only for a relatively short period of time. For example, a researcher is
studying the effects of music therapy on blood pressure of patients who are
admitted to the intensive care unit. Although blood pressure is measured
every hour for a 24-hour period, this study would not be described as a
longitudinal study.
Longitudinal designs provide important information about the chron-
ological relationships that exist between the IV and DV by determining
changes over time. Another advantage is that this type of design can be
used to test cause and effect. One disadvantage of longitudinal studies is
KEY TERM
crossover designs:
Experimental
designs that
use two or more
treatments; subjects
receive treatments
in random order6.3 Categorizing Designs According to Time 165