Statistical analysis
Many find the statistical analysis the most
difficult part of research. It is also the most commonly criticised part of
papers written by other clinicians.
There
are many useful books about statistics which can be consulted; if in any doubt a
statistician will be pleased to give assistance in analysing research.
Statisticians like to be consulted before research has been conducted rather
than being
presented with the results at the end of the
trial; they often give helpful advice over study design.
The
following terms are frequently used when summarising statistical data:
Mean: the
result of dividing the total by the number of observations (the average).
Median: the
middle value with equal number of observations above and below — used for
numerical or ranked data.
Mode: the
value with the highest frequency observed — used for nominal data collection.
Range: the
largest to the smallest value.
The
most important decision for analysis is whether the distribution of results is
normal (parametric). Normally distributed results have a symmetrical,
bell-shaped curve, and the mean, median and mode all lie at the same value. Some
types of data, such as blood group, are not normally distributed and require
other methods of analysis (nonparametric).
Parametric
tests
When the results are normally distributed a
t-test can be used to compare the outcome between intervention and control
groups. Confidence intervals are the best guide to the possible range in which
the true differences are likely to lie. A confidence interval that includes zero
usually implies a lack of statistical significance.
Nonparametric
tests
Statistical tests such as the chi-squared
test, Wilcoxon Signed Rank test (single sample) and the Mann—Whitney U-test
(compares two samples) can be used because they make no assumptions about the
underlying population distribution.
Scientists
usually employ P values to describe
statistical chance. A P value <0.05 is taken to imply a true difference. It is important not to
forget that P = 0.05 simply means
there is only a one in 20 chance that the differences between the variables
happened by chance. If enough variables are examined in any study significant
differences will occur simply due to chance.
Statistics
simply deal with the chance that observations between populations are different.
Clinical results should show clear differences. If statistics are required to
demonstrate differences between results, they are unlikely to have major
clinical significance.
Computer
software packages available
Statistical computer packages offer a quick
way to analyse descriptive statistics such as mean, median and the range, as
well as the most commonly used statistical tests such as the chi-squared test.
Various packages are available commercially and are useful tools in data
analysis.