Page 118 - Statistics for Dummies

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Evaluating a bar graph
one variable; make sure it’s clear what’s
To raise the statistical bar on bar graphs, check
out these tips:
being summarized.
✓ Bars that divide up values of a numerical
variable (such as income) should be equal
the percentage within each group (relative
frequencies) or the number in each group
in width (if possible) for fair comparison.
(frequencies).
✓ Be aware of the scale of the bar graph and
✓ If you see relative frequencies, check for
determine whether it’s an appropriate rep-
resentation of the information.
the total sample size — it matters. If you
see frequencies, divide each one by the
✓ Some bar graphs don’t sum to one because
total sample size to get percentages, which
they are showing the results of more than
are easier to compare.
Part II: Number-Crunching Basics ✓ Check whether the results are shown as
If you take a look at the percentages shown for each pet peeve listed, you see
they don’t sum to one. That tells you that each person surveyed was allowed
to choose more than one pet peeve (like that would be hard to do); perhaps
they were asked to name their top three pet peeves, for example. For this data
set and others like it that allow for multiple responses, a pie chart wouldn’t be
possible (unless you made one for every single pet peeve on the list).
Note that Figure 6-7 is a horizontal bar graph (its bars go side to side) as opposed
to a vertical bar graph (in which bars go up and down, as in Figure 6-6). Either ori-
entation is fine; use whichever one you prefer when you make a bar graph. Do,
however, make sure that you label the axes appropriately and include proper
units (such as gender, opinion, or day of the week) where appropriate.

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