Data collected in an experiment are often presented in the form of a graph, a diagram showing the relationships among the independent and dependent variable(s). The independent variable is usually graphed on the x-axis (horizontal axis, also known as the abscissa). The dependent variable is graphed on the y-axis (vertical axis, also known as the ordinate). Since a graph is a picture of the results, it can often be more easily interpreted than a table. By looking at a graph, then, you can visualize the effect that the independent variable has on the dependent variable. This may be one of the first steps in analyzing and interpreting your results.
When you are drawing a graph, keep in mind that your objective is to show the data in the clearest, most readable form possible. To achieve this, you should observe the following rules:
1. Use graph paper to plot the values accurately
2. Plot the independent variable on the x-axis and the dependent variable on the y-axis.
3. The intervals labeled on each axis should be appropriate for the range of data.
This allows for the most efficient use and also uses most of the area of the graph. For example, if the highest data point is 147, the highest value labeled on the axis might be 150. If you labeled intervals on up to 200, there would be a large unused area of the graph. Generally, begin both axes of the graph at zero (0) in the extreme left corner. To avoid generating graphs with wasted space, you may signify unused graph space by two vertical tic marks between the zero and your lowest number on one or both axes.
4. The intervals labeled on the graph should be evenly spaced. For example, if the values range from 0 to 50, you might label the axis at 0, 10, 20, 30, 40, and 50. It would be confusing to have labels that correspond to the actual data points (e.g. 2, 17, 24, 30, 42, 47).
5. Label each axis with the name of the variable and specify the units used to measure it. For example, the x-axis might be labeled Athlete, nonathlete," and the y-axis might be labeled "Pulse rate (beats/minute)."
6. Choose the type of graph that best presents your data. Line graphs and bar graphs are the most frequently used. The choice of graph type depends on the nature of the variable being graphed.
The Line Graph
Line graphs show changes in the quantity of the chosen variable and emphasize the rise and fall of the values over their range. For example, changes in the dependent variable pulse rate, measured over time, would be depicted best in a line graph. Use a line graph to present continuous data.
1. Plot data as separate points.
2. Generally, do not connect points dot to dot, but draw smooth curves or straight line to fit the values plotted for any one data set.
3. If more than one set of data is presented on a graph, provide a key to indicate which set is which.
The information is modified from: Investigating Biology: A Laboratory Manual for BIOLOGY. Morgan/Carter. Benjamin /Cumming’s Publishing Company, New York, 1993, pp.14-15.