Comparing graphs

We often need to analyze and compare cumulative graphs. Malott discusses the cumulative graph in Ch. 17 and Cooper, Heron, and Heward discuss it in Ch. 6. Three basic ways in which you can compare graphs are in terms of their level, trend, and variability.

The level of behavior depicted on a graph refers to the average frequency of the behavior across time. Calculate it by dividing the number of responses by the amount of time in which those responses were made. For example, if a rat pressed a lever 120 times in an hour, then one way to express the level of behavior would be 2 responses per minute (120 lever presses divided by 60 minutes). When visually analyzing a graph, imagine a straight horizontal line running across the graph at the level on the vertical axis that represents the average frequency based on all the data points. Sometimes a graph actually shows this "mean level line" (sometimes it's a "median level line"). The higher the mean level line is from the baseline, the greater the average frequency of the behavior across the period of time represented by the graph.

Trend refers to the overall tendency across time for the behavior to increase in frequency, decrease in frequency, or remain stable. Again, imagine a straight line, this time running through the data points in such a way that approximately half of them are above the line and half of them are below the line. If this trend line is horizontal, it tells you that, overall, the behavior did not change in frequency over the time period represented by the graph. If the line slopes upward from left to right, the frequency increased across time, and if it slopes downward, the frequency decreased. Another word for trend that you'll often see is slope.

Variability refers to the average change in frequency from one data point to the next. Imagine the trend line again. If the actual data points tend to be far from the line, then variability is high. This would indicate that the frequency of the behavior tended to change a lot from moment to moment during the session. There were periods of fast responding mixed in with periods of slow responding. We'd probably describe a line like this as very "jagged." When variability is low, the line is less jagged, more smooth.

The following graph shows a low level of behavior, a near-zero trend, and low or fairly stable variability.


The following graph shows a moderate level of behavior, an increasing trend, and high variability. Notice that a trend line has been added.


The following graph shows a low level of behavior, a slightly increasing trend, and moderate variability: