272 Part 4: The State of the World
How to Read It
Line graphs should have a title to tell you what information is being presented. Check that first,
then examine the scales. The horizontal scale should show time, and it should be clear what the
units of time are (years, months, days, etc.). This graph shows the average number of absences
due to illness at County High in the months from January to June.The range of numbers on the vertical scale and the units in which they are marked will depend
on the information being presented, but take the time to read it and understand the units. This
scale runs from 0 to 45, and the grid lines are every five units. Check for a broken scale and
remember that a broken scale may distort your perception of size, just like in a bar graph, so
examine the graph carefully. This scale is not broken, so there’s no concern.
The line segments connecting the points will help you see overall patterns, but don’t ignore
the dots themselves. They are the actual observed and recorded data. The average number of
absences due to illness at County High in May looks like 4. Connecting the points with lines is
based on the assumption that the change from point to point is smooth and consistent, and that’s
not always true. Be very cautious about making any predictions or drawing any conclusions from
what the line segments are doing. For example, the lines in this graph suggest that the sharpest
decline in absentees took place from February to March, and that there were increases from
January to February and from May to June. You wouldn’t try to use the lines to say that there
were 15 absentees on a certain date, however, because the line segments don’t represent recorded
data, only trends.45
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5
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Jan Feb Mar Apr May JunAbsences Due to Illness