Graphing Requirements

Students will usually use computer software to create their graphs. In so doing, they should keep in mind the following elements of effective graphing:

· A graph must have a title that informs the reader about the experiment and tells the reader exactly what is being measured.

· The reader should be able to easily identify each line or bar on the graph.

· Axes must be clearly labeled with units as follows:

Ø The x-axis shows the independent variable. Time is an example of an independent variable. Other possibilities for an independent variable might be light intensity or the concentration of a hormone or nutrient.

Ø The y-axis denotes the dependent variable—the variable that is being affected by the condition (independent variable) shown on the x-axis.

Ø Intervals must be uniform. For example, if one square on the x-axis equals five minutes, each interval must be the same and not change to 10 minutes or one minute. If there is a break in the graph, such as a time course over which little happens for an extended period, it should be noted with a break in the axis and a corresponding break in the data line.

Ø It is not necessary to label each interval. Labels can identify every five or 10 intervals, or whatever is appropriate.

Ø The labels on the x-axis and y-axis should allow the reader to easily see the information.

· More than one condition of an experiment may be shown on a graph by the use of different lines. For example, the appearance of a product in an enzyme reaction at different temperatures can be compared on the same graph. In this case, each line must be clearly differentiated from the others—by a label, a different style, or colors indicated by a key. These techniques provide an easy way to compare the results of experiments.

· The graph should clarify whether the data start at the origin (0,0) or not. The line should not be extended to the origin if the data do not start there. In addition, the line should not be extended beyond the last data point (extrapolation) unless a dashed line (or some other demarcation) clearly indicates that this is a prediction about what may happen.

· *For most of the labs in the lab manual, students should include standard error in their analysis and use standard error bars on their graphical displays when appropriate.