|Part of a series on Statistics|
In statistics, a misleading graph, also known as a distorted graph, is a graph which misrepresents data, constituting a misuse of statistics and with the result that an incorrect conclusion may be derived from it.
Graphs may be misleading through being excessively complex or poorly constructed. Even when constructed to accurately display the characteristics of their data, graphs can be subject to different interpretation.
Misleading graphs may be created intentionally to hinder the proper interpretation of data or accidentally due to unfamiliarity with graphing software, misinterpretation of data, or because data cannot be accurately conveyed. Misleading graphs are often used in false advertising. One of the first authors to write about misleading graphs was Darrell Huff, publisher of the 1954 book How to Lie with Statistics.
The field of data visualization describes ways to present information that avoids creating misleading graphs.
- 1 Misleading graph methods
- 2 Measuring distortion
- 3 Usage in finance and corporate reports
- 4 Academia
- 5 See also
- 6 References
- 7 Further reading
- 8 External links
Misleading graph methods
There are numerous ways in which a misleading graph may be constructed.
The use of graphs where they are not needed can lead to unnecessary confusion/interpretation. Generally, the more explanation a graph needs, the less the graph itself is needed. Graphs do not always convey information better than tables.
- Comparing pie charts of different sizes could be misleading as people cannot accurately read the comparative area of circles.
- The usage of thin slices which are hard to discern may be difficult to interpret.
- The usage of percentages as labels on a pie chart can be misleading when the sample size is small.
- Making a pie chart 3D or adding a slant will make interpretation difficult due distorted effect of perspective. Bar-charted pie graphs in which the height of the slices is varied may confuse the reader.
3D Pie chart slice perspective
A perspective (3D) pie chart is used to give the chart a 3D look. Often used for aesthetic reasons, the third dimension does not improve the reading of the data; on the contrary, these plots are difficult to interpret because of the distorted effect of perspective associated with the third dimension. The use of superfluous dimensions not used to display the data of interest is discouraged for charts in general, not only for pie charts. In a 3D pie chart, the slices that are closer to the reader appear to be larger than those in the back due to the angle at which they're presented.
|Misleading Pie Chart||Regular Pie Chart|
|In the misleading pie chart, Item C appears to be at least as large as Item A, whereas in actuality,
it is less than half as large.
Tables are preferable to graphics for many small data sets. A table is nearly always better than a dumb pie chart; the only thing worse than a pie chart is several of them, for then the viewer is asked to compare quantities located in spatial disarray both within and between pies - Given their low data-density and failure to order numbers along a visual dimension, pie charts should never be used.
When using pictogram in bar graphs, they should not be scaled uniformly as this creates a perceptually misleading comparison. The area of the pictogram is interpreted instead of only its height or width. This causes the scaling to make the difference appear to be squared.
|Note how in the improperly scaled pictogram bar graph, the image for B is actually 9 times as large as A.|
|Note how the perceived size increases when scaling.|
The effect of improper scaling of pictogram is further exemplified when the pictogram has 3 dimensions, in which case the effect is cubed.
|Note how the usage of improper scaling of a three-dimensional pictogram in this fictitious graph. It appears that home sales have gone up significantly in 2001 over the previous year. Additionally, because no frequency axis is supplied, readers are unable to quantify the change, and are only left with a misleading perception of the change. The scaling, which is 2x, causes the change to appear to be or 8 times as large.|
Additionally, an improperly scaled pictogram may leave the reader with the sense that the item itself has actually changed in size.
|Assuming the pictures represent equivalent quantities, note how in the misleading graph, there appears to be more bananas
because the bananas occupy the most area and are furthest to the right.
A truncated graph (also known as a torn graph) has a y-axis that does not start at 0. These graphs can create the impression of important change where there is relatively little change.
Truncated graphs are useful in illustrating small differences. Graphs may also be truncated to save space. Commercial software such as MS Excel will tend to truncate graphs by default if the values are all within a narrow range, as in this example.
|There are several ways to indicate a y-axis break.|
|Original graph||Half width, twice height||Twice width, half height|
|Changing the ratio of a graph's dimensions will affect how the graph appears.|
|Less difference||More difference|
|Note the lack of a starting value for the y-axis, which makes it unclear if the graph is truncated. Additionally, note the lack of tick marks which prevents the reader from determining if the graph bars are properly scaled. Without a scale, the visual difference between the bars can be easily manipulated.|
|Volatility||Steady, fast growth||Slow growth|
|Though all three graphs share the same data, and hence the actual slope of the (x,y) data is the same, the way that the data is plotted can change the visual appearance of the angle made by the line on the graph. This is because each plot has different scale on its vertical axis. Because the scale is not shown, these graphs can be misleading.|
The intervals and units used in a graph may be manipulated to create or mitigate the expression of change.
Graphs created with omitted data remove information from which to base a conclusion.
|Scatter plot with missing categories||Regular scatter plot|
|Note how in the scatter plot with missing categories on the left,
the growth appears to be more linear with less variation.
In financial reports, negative returns, or data which does not correlate a positive outlook may be excluded to create a more favorable visual impression.
Graphs based on other graphs should be representative in their presentation.
Extraction has valid uses when searching for anomalies.
|Original Graph||Extracted Graph|
|Note how the extracted graph does not accurately represent the original graph.|
The use of a superfluous third dimension which does not contain information is strongly discouraged as it may confuse the reader.
The third dimension may confuse readers.
Graphs are designed to allow for easier interpretation of statistical data. However, graphs with excessive complexity can obfuscate the data and make interpretation difficult.
Poorly constructed graphs can make data difficult to discern and thus interpret.
A graph with a high lie factor (>1) would exaggerate change in the data it represents, while one with a small lie factor (>0, <1) would obscure change in the data. A perfectly accurate graph would exhibit a lie factor of 1.0.
Graph discrepancy index
The graph discrepancy index also known as the graph distortion index (GDI) was originally proposed by Paul John Steinbart in 1998. GDI is calculated as a percentage ranging from -100% to positive infinity with zero percent indicating that the graph has been properly constructed and anything outside the ±5% margin is considered to be distorted. Research into the usage of GDI as a measure of graphics distortion has found it to be inconsistent and discontinuous making the usage of GDI as a measurement for comparisons difficult.
The data-ink ratio should be relatively high, otherwise the chart may have unnecessary graphics.
The data density should be relatively high, otherwise a table may be better suited for displaying the data.
Usage in finance and corporate reports
Graphs are useful in the summary and interpretation of financial data. Graphs allow for trends in large data sets to be seen while also allowing the data to be interpreted by non-specialists.
Graphs are often used in corporate annual reports as a form of impression management. In the United States, graphs do not have to be audited as they fall under AU Section 550 Other Information in Documents Containing Audited Financial Statements.
Several published studies have looked at the usage of graphs in corporate reports for different corporations in different countries and have found frequent usage of improper design, selectivity, and measurement distortion within these reports. The presence of misleading graphs in annual reports have led to requests for standards to be set.
Research has found that while readers with poor levels of financial understanding have a greater chance of being misinformed by misleading graphs, even those with financial understanding, such as loan officers, may be misled.
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