Statistical Methods in Medical Research 4ed

2 The scope of statistics
other patient. The clinical judgement exercised by a physician in the choice of
treatment for an individual patient is based to an extent on theoretical considerations
derived from an understanding of the nature of the illness. But it is
based also on an appreciation of statistical information about diagnosis, treatment
and prognosis acquired either through personal experience or through
medical education. The important argument is whether such information should
be stored in a rather informal way in the physician's mind, or whether it should
be collected and reported in a systematic way. Very few doctors acquire, by
personal experience, factual information over the whole range of medicine, and it
is partly by the collection, analysis and reporting of statistical information that a
common body of knowledge is built and solidified.
The phrase evidence-based medicine is often applied to describe the compilation
of reliable and comprehensive information about medical care (Sackett et
al., 1996). Its scope extends throughout the specialties of medicine, including, for
instance, research into diagnostic tests, prognostic factors, therapeutic and prophylactic
procedures, and covers public health and medical economics as well as
clinical and epidemiological topics. A major role in the collection, critical evaluation
and dissemination of such information is played by the Cochrane Collaboration,
an international network of research centres (http://www.cochrane.org/).
In all this work, the statistical approach is essential. The variability of disease
is an argument for statistical information, not against it. If the bedside physician
finds that on one occasion a patient with migraine feels better after drinking
plum juice, it does not follow, from this single observation, that plum juice is a
useful therapy for migraine. The doctor needs statistical information showing,
for example, whether in a group of patients improvement is reported more
frequently after the administration of plum juice than after the use of some
alternative treatment.
The difficulty of arguing from a single instance is equally apparent in studies
of the aetiology of disease. The fact that a particular person was alive and well at
the age of 95 and that he smoked 50 cigarettes a day and drank heavily would not
convince one that such habits are conducive to good health and longevity.
Individuals vary greatly in their susceptibility to disease. Many abstemious
non-smokers die young. To study these questions one should look at the morbidity
and mortality experience of groups of people with different habits: that is,
one should do a statistical study.
The second chapter of this book is concerned mainly with some of the basic
tools for collecting and presenting numerical data, a part of the subject usually
called descriptive statistics. The statistician needs to go beyond this descriptive
task, in two important respects. First, it may be possible to improve the quality
of the information by careful planning of the data collection. For example,
information on the efficacy of specific treatments is most reliably obtained
from the experimental approach provided by a clinical trial (Chapter 18),