Statistical Methods in Medical Research 4ed

6 The scope of statistics
pressure were an exact function of age, varying neither from person to person
nor between occasions on the same person, the blood pressure at age 55 could be
determined by one observation only. Such studies would not be statistical in
nature and would not call for statistical analysis. Those situations, of course, do
not hold. The duration of symptoms from the common cold varies from one
attack to another; blood pressures vary both between individuals and between
occasions. Comparisons of the effects of different medical treatments must
therefore be made on groups of patients; studies of physiological norms require
population surveys.
In the planning of a statistical study a number of administrative and technical
problems are likely to arise. These will be characteristic of the particular field of
research and cannot be discussed fully in the present context. Two aspects of the
planning will almost invariably be present and are of particular concern to the
statistician. The investigator will wish the inferences from the study to be
sufficiently precise, and will also wish the results to be relevant to the questions
being asked. Discussions of the statistical design of investigations are concerned
especially with the general considerations that bear on these two objectives.
Some of the questions that arise are: (i) how to select the individuals on which
observations are to be made; (ii) how to decide on the numbers of observations
falling into different groups; and (iii) how to allocate observations between
different possible categories, such as groups of animals receiving different treatments
or groups of people living in different areas.
It is useful to make a conceptual distinction between two different types of
statistical investigation, the experiment and the survey. Experimentation involves
a planned interference with the natural course of events so that its effect can be
observed. In a survey, on the other hand, the investigator is a more passive
observer, interfering as little as possible with the phenomena to be recorded. It is
easy to think of extreme examples to illustrate this antithesis, but in practice the
distinction is sometimes hard to draw. Consider, for instance, the following series
of statistical studies:
1 A register of deaths occurring during a particular year, classified by the cause
of death.
2 A survey of the types of motor vehicle passing a checkpoint during a certain
period.
3 A public opinion poll.
4 A study of the respiratory function (as measured by various tests) of men
working in a certain industry.
5 Observations of the survival times of mice of three different strains, after
inoculation with the same dose of a toxic substance.
6 A clinical trial to compare the merits of surgery and conservative treatment
for patients with a certain condition, the subjects being allotted randomly to
the two treatments.