Graphs That Lie

Graphs That Lie ~ Issues of Scale

Which graphs(s) on this page tell the truth?

Fig . 1

Fig. 2

Fig. 3

So, what's the problem here?

Fig 1. gives the most accurate picture of the data.

Fig 2. is also accurate it's just that it shows a zoomed in detailed view. Right? The problem is that Fig 2. is misleading. At first glance it appears that the extraterrestrial population of Erin has tripled in the last 50 years. What makes it look that way? The scale is the issue: it exaggerates the change because it doesn't start at zero. So the difference between the blue 1950 bar and the red 2000 bar appears more significant than it actually is. But Fig 1. shows us that in fact there has been a relatively minor increase.

Fig 3. has a similar effect simply by stretching the height of the graphic thereby exaggerating the vertical axis: once again the change over time appears larger.

Fig. 4

So, what's the problem?

Again, the issue is the scale. This time the problem is that the scale is not consistent. Why does this matter? Well, at first glance Fig. 4 makes it look like the increase in births between 1990 and 1992 is the same as the increase in births from 1998 to 2000: the line length is the same and it goes up the same number of units. In fact the increase from 1998 to 2000 (750 births) is five times greater than the increase from 1990 to 1992 (150 births). The reason they look the same is because the scale is inconsistent: at first the vertical axis increases by a unit of 50 but at 250 it switches to a unit of 250. So, even though both segments go up 3 lines, the 1998-2000 segment represents a significantly larger increase.

The units on a graph must remain consistent.

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