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Bar charts

Bar charts can also be used to represent frequencies ofdifferent categories. A bar chart of the iMac purchases is shown in . Frequencies are shown on the Y axis and the type of computer previously owned is shown on theX axis. Typically the Y-axis shows the number of observations rather than the percentage of observations in each category asis typical in pie charts.

Bar chart of iMac purchases as a function of previous computer ownership.

Comparing distributions

Often we need to compare the results of different surveys,or of different conditions within the same overall survey. In this case, we are comparing the distributions of responses between the surveys or conditions. Bar charts are often excellent forillustrating differences between two distributions. shows the number of people playing card games at the Yahoo website on a Sunday and on a Wednesday on a day in the Spring of 2001. We see that there were moreplayers overall on Wednesday compared to Sunday. The number of people playing Pinochle was nonetheless the same on thesetwo days. In contrast, there were about twice as many people playing hearts on Wednesday as on Sunday. Facts like theseemerge clearly from a well-designed bar chart.

A bar chart of the number of people playing different card games on Sunday and Wednesday.
The bars in are oriented horizontally rather than vertically. The horizontal format is useful whenyou have many categories because there is more room for the category labels. Such horizontal bar charts may be contrasted with vertical bar charts like the one in . We'll have more to say about bar charts when we consider numerical quantities later in this chapter. (See Bar Charts .)

Some graphical mistakes to avoid

Don't get fancy! People sometimes add features to graphs that don't help to convey their information. For example,3-dimensional bar charts like the one shown in are usually not as effective as their two-dimensional counterparts.

A three-dimensional version of .
Here is another way that fanciness can lead to trouble. Instead of plain bars, it is tempting to substitutemeaningful images. For example, presents the iMac data using pictures of computers. The heights of thepictures accurately represent the number of buyers, yet is misleading because the viewer's attention will be captured by areas. This can exaggerate the sizedifferences between the groups. In terms of percentages, the ratio of previous Macintosh owners to previous Windows ownersis about 6 to 1. But the ratio of the two areas in is about 35 to 1. A biased person wishing to hide the fact that many Windows owners purchased iMacs wouldbe tempted to use instead of ! Edward Tufte coined the term lie factor to refer to the ratio of the size of the effect shown in a graph to the size of the effect shown in thedata. He suggests that lie factors greater than 1.05 or less than 0.95 produce unacceptable distortion.
A redrawing of with a lie factor greater than 8.
Another distortion in bar charts results from setting the baseline to a value other than zero. The baseline is thebottom of the Y-axis, representing the least number of cases that could have occurred in a category. Normally, this numbershould be zero. shows the iMac data with a baseline of 50. Once again, the difference in areas suggestsa different story than the true differences in percentages. The number of Windows-switchers seems minusculecompared to its true value of 12%.
A redrawing of with a baseline of 50.
Finally, we note that it is a serious mistake to use a line graph when the X-axis contains merely qualitative variables. Aline graph is essentially a bar graph with the tops of the bars represented by points joined by lines (the rest of the bar issuppressed). inappropriately shows a line graph of the card game data from Yahoo. The drawback to is that it gives the false impression that the games are naturally ordered in a numerical way.

A line graph of the number of people playing different card games on Sunday and Wednesday.

Summary

Pie charts and bar charts can both be effective methods of portraying qualitative data. Bar charts are better when thereare more than just a few categories and for comparing two or more distributions. Be careful to avoid creating misleadinggraphs.

Questions & Answers

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RAW Reply
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The nanotechnology is as new science, to scale nanometric
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nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
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what king of growth are you checking .?
Renato
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
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yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
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biomolecules are e building blocks of every organics and inorganic materials.
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Source:  OpenStax, Collaborative statistics (custom online version modified by t. short). OpenStax CNX. Jul 15, 2013 Download for free at http://cnx.org/content/col11476/1.5
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