On 16 Aug 2001, John Uebersax asked for software "that produces
publication quality Venn diagrams":
> I want something to summarize and communicate to non-statisticians
> (e.g., physicians) the overlap between two sets (such as patients who
> have Major Depression those who receive antidepressant meds).
Do you have reason to believe that your clients are particularly familiar
with, and accustomed to interpreting, Venn diagrams? If not, why not use
a simple two-way table of frequencies (or proportions)? This has the
possible virtue of being readily extensible to three or more sets,
whereas the characteristics you ask for below can be guaranteed only for
two sets in Venn diagrams (and even then not for the complementary space
representing the elements that belong to neither set).
> The diagram should show the area of each circle as proportional [to]
> its N, and the overlap area as proprotional to the number of cases in
> both groups.
Venn diagrams don't strictly need to be displayed in terms of circles;
it's merely customary, or perhaps conventional. (Possibly because rough
circles are easier to draw on a blackboard in more or less recognizable
form than squares or rectangles.) The geometric task would be easier if
you used squares, for which this kind of proportionalitity is fairly
easy to arrange (and construct). Of course, in no case can you manage
to get the area of the circles (or squares, or whatever figures please
you) to be proportional to their respective N's *and* have the area of
the complementary set (those that are neither 'A' nor 'B') proportional
to its N, unless the complementary set is rather large in comparison to
'A' and 'B'.
It would be possible to subdivide a square or rectangular space into four
subsets whose areas are proportional as described; but I do not think
one could guarantee that more than three of the four subsets would be
rectangular (the fourth might be L-shaped), nor that the sets 'A' and
'B' (both of which contain 'AB') would both be rectangular.
Tables are more general, and in some senses simpler (the subspaces are
all rectangular, you can display 'A' and 'B' with differently colored
outlines, and their intersection is obvious). But perhaps this approach
would not be viable, if you happen to be dealing with numerophobes for
clients. (OTOH, the *logical* relationships are fairly clear, and one
can always avoid talking about the actual *numbers* involved.)
------------------------------------------------------------------------
Donald F. Burrill [EMAIL PROTECTED]
184 Nashua Road, Bedford, NH 03110 603-471-7128
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