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In-Depth Information
[1987] uncovered an average attrition rate of 47% in studies lasting 4 to
10 years.
You need to analyze the data to ensure that the proportions of missing
observations are the same in all treatment groups. Again, traditional statis-
tical methods are applicable only if missing data are not treatment related.
Deaths and disabling accidents and diseases, whether or not directly
related to the condition being treated, are common in long-term trials in
the elderly and high-risk populations. Or individuals are simply lost to
sight (“no forwarding address”) in highly mobile populations.
Lang and Secic [1997, p. 22] suggest a chart such as that depicted in
Figure 3.1 as the most effective way to communicate all the information
regarding missing data. Censored and off-scale measurements should be
described separately and their numbers indicated in the corresponding
tables.
TABLES
Is text, a table, or a graph the best means of presenting results? Dyke
[1997] would argue, “Tables with appropriate marginal means are often the
best method of presenting results, occasionally replaced (or supplemented)
by diagrams, usually graphs or histograms.” Van Belle [2002] warns that
aberrant values often can be more apparent in graphical form. Arguing in
favor of the use of ActivStats
®
for exploratory analysis is that one can so
easily go back and forth from viewing the table to viewing the graph.
A sentence structure should be used for displaying two to five numbers,
as in “The blood type of the population of the United States is approxi-
mately 45% O, 40% A, 11% B, and 4% AB.”
3
Note that the blood types
are ordered by frequency.
Marginal means may be omitted only if they have already appeared in
other tables.
4
Sample sizes should always be specified.
Among our own worst offenses is the failure to follow van Belle's advice
to “Use the table heading to convey critical information. Do not stint.
The more informative the heading, the better the table.”
5
Consider adding a row (or column, or both) of contrasts; “for example,
if the table has only two rows we could add a row of differences, row 1
minus row 2: if there are more than two rows, some other contrast might
be useful, perhaps 'mean haploid minus mean diploid', or 'linear compo-
nent of effect of N-fertilizer'.”
6
Indicate the variability of these contrasts.
3
van Belle [2002, p. 154].
4
Dyke [1997]. Reprinted with permission from Elsevier Science.
5
van Belle [2002, p. 154].
6
Dyke [1997]. Reprinted with permission from Elsevier Science.
