Topic Sections

## Drawing graphs

Graphs are a visual way of showing the relationship between two variables. It is much easier to pick out relationships between different variables from a graph than from a table.

The following guidelines should be remembered when constructing tables:

• the title should describe the relationship being investigated;
• plot the independent variable on the x-axis and the dependent variable (ie not controlled by the experimenter) on the y-axis;
• label the axes with names of quantities being measured and their units;
• choose scales so that the page is filled;
• only include the origin if specifically needed. Usually this would be if the range of data included or went close to the origin. Intercepts of straight lines can be calculated much more accurately than they can be read from a graph;
• choose simple scales: factors of two, five and 10 are much easier than three or seven. The latter scales make points very difficult to plot and are far more likely to lead to mistakes;
• plot data points using an appropriate shape or symbol such as an x or dots;
• add range bars to show the error in the data; and
• join the points with the smoothest curve possible so that half the points lie above and half below the line. The term 'curve' is used here in its most general sense, this will be a straight line in certain cases. No departure from a smooth curve should be accepted unless there are several neighbouring points supporting this course of action.

Tips for teaching

1. Students need plenty of practice at drawing graphs, make sure that the curriculum plan provides lots of opportunities.

2. Present the class with a table of data and a graph full of mistakes and ask them to spot the errors..

3. Make use of peer assessment. Students often respond well to feedback from their peers.

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