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Dependent and Independent Variables

Variables can be classified as dependent or independent. When there is a linear relationship between two variables, the values of one variable depend upon the values of the other variable.

Data is often recorded in a table format called a table of values. Since most graphs compare two variables, the table of values will have two columns of numerical values.

The concentration of oxygen decreases at higher altitudes as shown in the table of values below:

Location	Altitude (metres)	Oxygen (%)
Vancouver	0	20.9
Edmonton	670	19.5
Calgary	1 050	18.7
Banff	1 300	18.2
Mount Ford, Rocky Mountains	3 500	13.7

The purpose of the first column in this table of values is to provide information. The second and third columns will be used for the graph since these columns contain numerical values.

Since the percentage of oxygen available is dependent on the altitude, the percentage of oxygen is the dependent variable and the altitude is the independent variable.

Graphs that are drawn on a grid have two axes labelled to represent the independent variable and the dependent variable. The independent variable, which can be chosen freely, is independent of the other variable and is drawn on the x-axis (horizontal axis). The dependent variable, which is not chosen freely is drawn on the y-axis (vertical axis) since it responds to or is dependent upon the independent variable.


If a variable can take on any value between two specified points, it is called a continuous variable; otherwise, it is called a discrete variable. In the graph above, it is possible for the altitude of a location to be 500 m, 1 800 m or any value between the plotted points so the altitude is a continuous variable. Since the graph is continuous, the points on the graph are connected to include all the values between the points. If the variable is discrete, the points are not connected and the graph will have various points.


Note: The graph above is linear since it forms a straight line.

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