Page 143 - Statistics for Dummies

Average Time between Old Faithful Eruptions
by Day They Were Observed
74
73
Figure 7-15:
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Time chart
showing
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daily aver-
age time
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between
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eruptions
for Old
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Faithful gey-
ser (n = 16 Time (minutes) 75 Chapter 7: Going by the Numbers: Graphing Numerical Data 127
67
consecutive
2 4 6 8 10 12 14 16
days). Day

A time chart condenses all the data for one unit of time into a single point. By
contrast, a histogram displays the entire sample of data that was collected at
that one unit of time. For example, Figure 7-15 shows the daily average time
between eruptions for 16 days. For any given day, you can make a histogram
of all the eruptions observed on that particular day. Displaying a time chart of
average times over 16 days accompanied by a histogram summarizing all the
eruptions for a particular day would be a great one-two punch.
Evaluating time charts
Here is a checklist for evaluating time charts, ✓ Watch for gaps in the timeline on a time
with a couple more thoughts added in: chart. For example, it’s misleading to show
equally spaced points on the horizontal
✓ Examine the scale and start/end points on
the vertical axis (the one showing the values (time) axis for 1990, 2000, 2005, and 2010.
of the data). Large increments and/or lots of This happens when years are just treated
white space make differences look less dra- like labels, rather than real numbers.
matic; small increments and/or a plot that ✓ As with any graph, take the units into
totally fills the page exaggerate differences. account; be sure they’re appropriate for
comparison over time. For example, are
✓ If the amount of data you have is over-
whelming, consider boiling it down by find- dollar amounts adjusted for inflation? Are
ing means/medians for blocks of time and you looking at number of crimes, or the
plotting those instead. crime rate?

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