| Module 2: Describing, Clarifying and Presenting Data
3. Data Displays
3.5. Reading a graphic display
Sometimes people assume that a graph or a chart is better than text because it is a picture. A picture is said to be worth a thousand words, but is it? What is known about reading graphs?
There is a story in every chart or graph
Remember that charts and graphs are telling you a story. When looking at a graph or a chart, you need to ask yourself, “What is the story that they are trying to tell?” In working out the story you are interpreting the chart or graph.
What do you need to know to interpret a chart or graph?
Research[13] has shown that most people find it easiest to interpret picture charts or pictographs, followed by circle graphs or pie charts, then bar charts and histograms. People found line graphs the most difficult to interpret. However, as was indicated earlier in this section, pictographs are only effective if each symbol has a stated value.
Interpreting line graphs
Many of the graphs that you look at in the media are line graphs that are designed to show trends in data. Because people find this type of graph such a challenge to interpret, the following sections will examine what you need to know to use such graphs successfully.
Line graphs use two reference axes (‘axis’ is the singular form of the word). One is vertical, the other horizontal. They provide reference scales for the data and/or function plotted on the graph.
Note that what we call a line graph (also called a ‘polygon’) may have curved sections as well as straight sections. Let’s see how well you are able to read a line graph.
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| Test your knowledge
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| Question
When is Pam heavier than Maurice?
- Never
- 13 years
- Between 11 and 18 years
- Once
| | Answer
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| Test your knowledge
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When is Pam growing faster than Maurice?
- Between 3 and 9 years
- From 0 to 3 years
- Between 10 and 15 years
- From 17 to 20 years
| | Answer
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c) Between 10 and 15 years. If Pam is growing faster than Maurice, then the slope of the line will be steeper for Pam than for Maurice. Hence, the answer is c).
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Read graphs like a sentence
To make sense of many line graphs you need to read them like you would a sentence, from left to right. Some people find that their eyes are drawn to something in the graphic display that stands out so they find it difficult to appreciate or understand the whole pattern that is shown in the graph. For example, look at the graph below. This is a graph of the effect of the decline in rabbit populations on the fox population in inland Australia.
What story is this graph telling you? What do you need to know to make sense of these data?
I. It helps to know something of the context; i.e. to know that foxes eat rabbits.
II. Identify what is represented on the vertical axis [Did you say population?] and what is represented on the horizontal axis [Did you say time?].
III. You then need to look at the relationship between the two curves. If you read the curves from left to right, what do you notice? Did you notice that the fox curve seems to follow the rabbit curve? This indicates that the fox population is affected by changes in the rabbit population. This would make sense because you were told that foxes feed on rabbits.
The three steps to reading a graph
When reading a graph, follow these steps:
I. Read the information that is provided with the graph because this tells you the context from which the graph was developed.
II. Identify the variables on the vertical and horizontal axes because this will help you to interpret the graph.
III. Read the graph from the beginning of the curve or curves at the vertical axis to the end and propose a relationship between the variables or between the curves. This will enable you to tell the story of the graph and interpret trends.
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| Test your knowledge
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| Question
What story do the two plots on this graph tell?
- There was a gradual increase in the amount of daisy plants under the tree.
- The light increased when the tree was pruned.
- The daisy plants grow taller because of the increased amount of light reaching the ground after the tree has been pruned and then grow shorter as the amount of light reaching the ground becomes less with the regrowth of leaves on the tree.
- Before the tree was pruned, the amount of light under the tree was constant, as was the amount of daisy plants. When the tree was pruned the amount of light that reached the ground increased so that more daisies grew but as the leaves of the tree grew again the amount of light on the ground returned to levels similar to before the pruning.
| | Answer
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