Section 2.4 Graphs Can Be Misleading 75 Table 2.20 Passenger Traffic at Denver International Airport Number of Passengers Year (in millions) 2002 35.7 2003 37.5 2004 42.4 2005 43.4 2006 47.3 2007 49.9 2008 51.2 Source: Wikipedia In order to get a better picture of the data, we can make a bar graph. Figures 2.19 and 2.20 present two different bar graphs of the same data. Figure 2.19 presents a clear picture of the data.We can see that the number of passengers has been rising gradually over the years. Now imagine that someone was eager to persuade us that passenger traffic had been increasing rapidly over the years. If they were to show us Figure 2.19, we wouldn’t be convinced. So they might show us a misleading picture like Figure 2.20 instead. Figure 2.20 gives the impression of a truly dramatic increase. 60 50 40 30 20 10 0 Millions of Passengers Passenger Traffic at Denver International Airport 2002 2003 2004 2005 2006 2007 2008 Figure 2.19 The bottom of the bars is at zero. This bar graph gives a correct impression of the data. 60 55 50 45 40 35 Millions of Passengers Passenger Traffic at Denver International Airport 2002 2003 2004 2005 2006 2007 2008 Figure 2.20 The bottom of the bars is not at zero. This bar graph exaggerates the differences between the bars. Figures 2.19 and 2.20 are based on the same data. Why do they give such different impressions? The reason is that the baseline (the value corresponding to the bottom of the bars) is at zero in Figure 2.19, but not at zero in Figure 2.20. This exaggerates the differences between the bars. For example, in Figure 2.20, the bar for the year 2007 is more than 10 times as long as the bar for the year 2002, but the actual increase in passenger traffic is much less than that. This sort of misleading information can be created with time-series plots as well. Figures 2.21 and 2.22 present two different time-series plots of the data. In Figure 2.21, the 2002 2003 2004 2005 2006 2007 2008 60 50 40 30 20 10 0 Millions of Passengers Passenger Traffic at Denver International Airport Figure 2.21 The baseline is at zero. This plot gives an accurate picture of the data. 2002 2003 2004 2005 2006 2007 2008 60 55 50 45 40 35 Millions of Passengers Passenger Traffic at Denver International Airport Figure 2.22 The baseline is not at zero. This plot exaggerates the rate of increase.
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