| The object of most studies is to prove a casual relationship between two
variables; that is, that a change in one variable "causes" a change in the
other. For example, one might find a causal relationship between amount of
exercise and weight loss. Another type of relationship besides the
casual one exits--that of 'association.' An associative relationship is
not necessarily causal, but can be explained by the presence of other 'un-seen'
variables to which the two variables being studied are themselves separately
linked. For instance, using our previous example, one might observe that
people who eat energy bars tend to lose weight at a greater rate that those who
do not. Although one might suspect a causal relationship between eating energy
bars and weight loss, it could also be argued that people who eat energy bars
also tend to exercise more and be health-conscience, which is the actual cause
of the weight loss. In this case energy bars and weight loss have an
associative relationship, but not a causal one. In scientific studies, the 'un-seen' variables are known as confounding
variables and their influence in eliminated by the presents of a control group.
For example, in our above study of the relationship between weight loss and
energy bars, exercise and healthy lifestyle would be the confounding variable.
To control for this confounder, we would try to make sure that our experimental
group and our control group each have the same number (or at least,
statistically insignificant differences in the number) of exercisers/healthy
lifestyles. Thus, any observed difference in weight loss between the two
groups at the end of the trial could be attributed to energy bars and not
exercise/lifestyle. Control is one of the greatest advantages of the scientific method over other
means of investigation that humans perform.
The difficulty with case-control studies is that they do not usually provide
convincing evidence of causal relationships. The reasons are as follows:
Case-control studies are so-called "retrospective" studies, which means that
they rely on data collected by looking over population records, usually several
years after cases have developed the disease, in order to elucidate a
relationship between two variables (in this case, amount of green tea ingested
and gastric cancer). Data, for instance, is frequently collected by means
of a questionnaire, asking people to estimate how many cups of green tea they
drank per day over a certain period, which may be 5-20 years ago--not a very
rigorous protocol. The data collection itself is subject to several
biases--e.g. errors in recall, personal prejudice. Secondly, retrospective
studies are not based on a designed
study, and as such they lack a control group (although this difficulty can be
overcome by artificially creating a control group by matching the 'test'
subjects with a similar population on potential confounders like age, sex, and
lifestyle). Often, case-control studies at best can simply point out an
association between two variables, and entice curious researchers to design
better studies to investigate a relationship more vigorously.
Researches often do attempt to investigate more vigorously, and the
next level of rigor is often the "prospective" study. In a prospective
study, the study is designed before the data is collected, which allows the
researchers several advantages over the retrospective study. Researchers
can design a study with a control group (the principle means of generating a
control group is randomization of people between the groups...this procedures
eliminates many confounding variables), administer a placebo (making the study
so called "placebo controlled"), design internal devices to eliminate bias (e.g.
"double-blind"), and
follow subject throughout the length of the study to observe development of the
outcome in question. The data generated from these studies is often
considered 'stronger' than data from retrospective studies, largely because of
the avoidance of confounding variables.
In respect to trials to determine the relationship of green tea consumption to gastric cancer, most of the initial data supporting an inverse relationship came from retrospective, case-controlled studies. Subsequent research with pre-designed and planned prospective studies have not agreed with the conclusion drawn from the retrospective investigations |
