Course 1.2Lesson 3 of 3
A histogram is for numerical data that has been put in groups, called class intervals. The bars touch, because the numbers continue from one group to the next, and the bar height is the frequency. The width of the groups changes the shape that we see. A line graph joins points over time, so that we can see a trend. A graph is misleading when its axis does not start at zero, or when it hides the scale.
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Marks are numbers. There are too many different marks to count one by one. So we put them in groups, called class intervals: 30 to 39, 40 to 49, and so on. Then we count the students in each group.
A histogram has one bar for each class interval. The bars touch, because the numbers continue from one group into the next.
The width of the groups is your choice, and it changes the picture. Too narrow, and most bars hold 0 or 1 values, so the shape is ragged. Too wide, and two or three bars hide the shape. Five to ten groups usually work well.
You should find that a width of 10 marks shows a clear hump around 70. A width of 5 marks gives many bars of 1 or 2. A width of 35 marks gives just two bars and tells you very little.
A line graph joins points that were measured one after another, such as a temperature every day. The line shows the trend: going up, going down or steady.
A graph can tell a false story.
The two graphs show the same data. On the right, the vertical axis starts at 49, so a small difference of 2 points looks like a huge gap. Always look at the numbers on the axis.
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
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