GRAPH AND CHARTS
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GRAPH AND CHARTS

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GRAPH AND CHARTS GRAPH AND CHARTS Document Transcript

  • Example 1 – Vertical bar graphsBar graphs should be used when you are showing segments of information. From theinformation given in the section on graph types, you will know that vertical bar graphsare particularly useful for time series data. The space for labels on the x-axis is small,but ideal for years, minutes, hours or months. At a glance you can see from the graphthat the scales for both the x- and y-axis increase as they get farther away from theorigin. Figure 1 below shows the number of police officers in Crimeville from 1993 to2001.In Figure 1 you can see that the number of police officers decreased from 1993 to 1996,but started increasing again in 1996. The graph also makes it easy to compare orcontrast the number of police officers for any combination of years. For example, in2001 there were nine more police officers than in 1998.The double (or group) vertical bar graph is another effective means of comparing sets ofdata about the same places or items. This type of vertical bar graph gives two or morepieces of information for each item on the x-axis instead of just one as in Figure 1. Thisallows you to make direct comparisons on the same graph by age group, sex, race, oranything else you wish to compare. However, if a group vertical bar graph has too manyseries of data, the graph becomes cluttered and it can be confusing to read.Figure 2, a double vertical bar graph, compares two series of data: the numbers of boysand girls using the Internet at Redwood Secondary School from 1995 to 2002. One barrepresents the number of boys who use the Internet and the other bar represents thegirls.
  • One disadvantage of vertical bar graphs, however, is that they lack space for textlabelling at the foot of each bar. When category labels in the graph are too long, youmight find a horizontal bar graph better for displaying information.Example 2 – Horizontal bar graphsThe horizontal bar graph uses the y-axis (vertical line) for labelling. There is more roomto fit text labels for categorical variables on the y-axis.Figure 3 shows the number of students at Diversity College who are immigrants by theirlast country of permanent residence. The graph shows that 100 students immigratedfrom China, 380 from France, and 260 from Brazil.A horizontal bar graph has been used to show a comparison of these data. This graph isthe best method to present this type of information because the labels (in this case, thecountries names) are too long to appear clearly on the x-axis.
  • A double or group horizontal bar graph is similar to a double or group vertical bargraph, and it would be used when the labels are too long to fit on the x-axis.In Figure 4, more than one piece of information is being delivered to the audience: druguse by 15-year-old boys is being compared with drug use by 15-year-old girls at Jamiesschool. Having both pieces of information on the same graph makes it easier tocompare. The graph indicates that 32% of boys and 29% of girls have used hashish ormarijuana, and 3% of boys and 1% of girls have tried LSD. The graph also shows thatthe same percent of boys and girls (4%) have used cocaine.Example 3 – Comparing several places or itemsFigure 5 is an example of a double horizontal bar graph. Hillary sampled an equalnumber of boys and girls at her high school and asked them to pick the one snack foodthey liked the most from the following list: • popcorn • chips • chocolate bars • crackers • pretzels • cookies • ice cream • fruit • candy • vegetables.She created a graph to display the results of her survey. Examine Figure 5, and answerthe following questions: 1. What comparison does this graph show? 2. Which snack food was least preferred by girls? 3. Which snack food was preferred by substantially more boys than girls? 4. Which snack foods were preferred by more girls than boys?
  • 5. Which snack food was preferred equally by both boys and girls?Answers 1. The graph shows a comparison of snack food preferences by sex. 2. Vegetables were the snack food least preferred by girls. 3. A substantial number of more boys than girls preferred chips. 4. Girls preferred candy, crackers, fruit and ice cream more than boys did. 5. The same number of boys and girls preferred popcorn as their snack food choice.Example 4 – Inappropriate use of bar graphsVertical bar graphs are an excellent choice to emphasize a change in magnitude. Thebest information for a vertical bar graph is data dealing with the description ofcomponents, frequency distribution and time-series statistics.A horizontal bar graph may be more effective than a line graph when there are fewertime periods or segments of data. If you want to compare more than 9 or 10 items, usea line graph instead. Figure 6 is an example of when a line graph should be usedinstead of a horizontal bar graph.
  • Example 5 – Other bar graphsThere are several other types of bar graphs that you may encounter. The populationpyramid is a special application of a double bar graph. The following examples are rarelyused, but can be useful if used correctly.Stacked bar graphsThe stacked bar graph is a preliminary data analysis tool used to show segments oftotals. Statistics Canada rarely uses them, despite the fact that stacked bar graphs canconvey a lot of information. The stacked bar graph can be very difficult to analyse if toomany items are in each stack. It can contrast values, but not necessarily in the simplestmanner.In Figure 7, it is not difficult to analyse the data presented since there are only threeitems in each stack: swimming, running and biking. It is easy to see at a glance whatpercentage of time each woman spent on an event. Had this been a graph representinga decathlon (with 10 events) the data would have been significantly harder to analyse.
  • Another reason that these graphs are rarely used is that they can represent a pictureother than the one that was intended. In the example above, it may have takenBronwyn two hours to finish the triathlon, and Rosalyn three hours, but they spentalmost the same percentage of time on each event. Both women spent 30% of theirtimes swimming, but whereas Rosalyn spent 54 minutes swimming, Bronwyn spent36 minutes swimming. In other words, this graph does not tell you anything about theirranking, only what percentage of their individual race times were spent on each event.This can be misleading for someone who does not read the graph carefully.Horizontal, vertical and stacked bar graph guidelinesYou should keep the following guidelines in mind when creating your own bar graphs: • Make bars and columns wider than the space between them. • Do not allow grid lines to pass through columns or bars. • Use a single font type on a graph. Try to maintain a consistent font style from graph to graph in a single presentation or document. Simple sans-serif fonts are preferable. • Order your shade pattern from darkest to lightest on stacked bar graphs. • Avoid garish colours or patterns.Dot graphsA dot graph is one of the simplest ways to represent information pictorially, yet it is thegraph that is least used. Figure 8 is an example of a dot graph. As you can see, themessage and the information behind the graph are delivered quickly and easily to thereader.
  • Pie charts • Constructing a pie chart • Pie charts versus bar graphsA pie chart is a way of summarizing a set of categorical data or displaying the differentvalues of a given variable (e.g., percentage distribution). This type of chart is a circledivided into a series of segments. Each segment represents a particular category. Thearea of each segment is the same proportion of a circle as the category is of the totaldata set.Pie charts usually show the component parts of a whole. Often you will see a segmentof the drawing separated from the rest of the pie in order to emphasize an importantpiece of information.
  • The pie chart above clearly shows that 90% of all students and faculty members atAvenue High School do not want to have a uniform dress code and that only 10% of theschool population would like to adopt school uniforms. This point is clearly emphasizedby its visual separation from the rest of the pie.The use of the pie chart is quite popular, as the circle provides a visual concept of thewhole (100%). Pie charts are also one of the most commonly used charts because theyare simple to use. Despite its popularity, pie charts should be used sparingly for tworeasons. First, they are best used for displaying statistical information when there areno more than six components only—otherwise, the resulting picture will be too complexto understand. Second, pie charts are not useful when the values of each componentare similar because it is difficult to see the differences between slice sizes.A pie chart uses percentages to compare information. Percentages are used becausethey are the easiest way to represent a whole. The whole is equal to 100%. Forexample, if you spend 7 hours at school and 55 minutes of that time is spent eatinglunch, then 13.1% of your school day was spent eating lunch. To present this in a piechart, you would need to find out how many degrees represent 13.1%. This calculationis done by developing the equation:percent ÷ 100 x 360 degrees = the number of degreesThis ratio works because the total percent of the pie chart represents 100% and thereare 360 degrees in a circle. Therefore 47.1 degrees of the circle (13.1%) represents thetime spent eating lunch.Constructing a pie chartA pie chart is constructed by converting the share of each component into a percentageof 360 degrees. In Figure 2, music preferences in 14- to 19-year-olds are clearly shown.
  • The pie chart quickly tells you that • half of students like rap best (50%), and • the remaining students prefer alternative (25%), rock and roll (13%), country (10%) and classical (2%).Tip! When drawing a pie chart, ensure that the segments are ordered by size (largestto smallest) and in a clockwise direction.In order to reproduce this pie chart, follow this step-by-step approach:If 50% of the students liked rap, then 50% of the whole pie chart (360 degrees) wouldequal 180 degrees.1. Draw a circle with your protractor.2. Starting from the 12 oclock position on the circle, measure an angle of 180 degrees with your protractor. The rap component should make up half of your circle. Mark this radius off with your ruler.3. Repeat the process for each remaining music category, drawing in the radius according to its percentage of 360 degrees. The final category need not be measured as its radius is already in position.Labeling the segments with percentage values often makes it easier to tell quickly whichsegment is bigger. Whenever possible, the percentage and the category label should beindicated beside their corresponding segments. This way, users do not have toconstantly look back at the legend in order to identify what category each colourrepresents.
  • The pie chart above conveys a clear message to the user—that 88% of all students inthe World Religions class celebrate Easter. We can easily tell what the message is bysimply looking at the accompanying percentages. Unfortunately, the category labels aretoo long to fit beside the pie segments, so they had to be placed in the legend. Ideally,these labels would also accompany the pie segments.It is more difficult to understand the message behind Figure 4 because there are nopercentage figures given for each slice of the pie. This is why it is important to label theslices with actual values.The user can still develop a picture of what is being said about the type of pets sold bythis store, but the message is not as clear as it would have been had the parts of thepie been labelled.In the pie chart above, the legend is formatted properly and the percentages areincluded for each of the pie segments. However, there are too many items in the piechart to quickly give a clear picture of the distribution of movie genres. If there aremore than five or six categories, consider using a another graph to display theinformation. Figure 5 would certainly be easier to read as a bar graph.
  • Tip! Many software programs will draw pie charts for you quickly and easily. However,research has shown that many people can make mistakes when trying to compare piechart values. In general, bar graphs communicate the same message with less chancefor misunderstanding.Pie charts versus bar graphsWhen displaying statistical information, refrain from using more than one pie chart foreach figure.Figure 6 shows two pie charts side-by-side, where a split bar graph (two bar graphsback-to-back) would have shown the information more clearly. A user might find itdifficult to compare a segment from one pie chart to the corresponding segment of theother pie chart. However, in a split bar graph, these segments become bars which arelined up back-to-back, making it much easier to make comparisons.Figure 7 shows how a split bar graph would be a better choice for displaying informationthan a double pie chart. The key point in preparing this type of graph is to ensure thatyou are using the same scale for both sides of the bar graph. Youll notice that theinformation is much clearer in Figure 7 than in Figure 6.
  • Line graphs • Comparing two related variables • Multiple line graphsLine graphs, especially useful in the fields of statistics and science, are more popularthan all other graphs combined because their visual characteristics reveal data trendsclearly and these graphs are easy to create.A line graph is a visual comparison of how two variables—shown on the x- and y-axes—are related or vary with each other. It shows related information by drawing acontinuous line between all the points on a grid. For information on the shapes of linegraphs, see the Organizing data chapter.Line graphs compare two variables: one is plotted along the x-axis (horizontal) and theother along the y-axis (vertical). The y-axis in a line graph usually indicates quantity(e.g., dollars, litres) or percentage, while the horizontal x-axis often measures units oftime. As a result, the line graph is often viewed as a time series graph. For example, ifyou wanted to graph the height of a baseball pitch over time, you could measure thetime variable along the x-axis, and the height along the y-axis.Although they do not present specific data as well as tables do, line graphs are able toshow relationships more clearly than tables do. Line graphs can also depict multipleseries and hence are usually the best candidate for time series data and frequencydistribution.Bar and column graphs and line graphs share a similar purpose. The column graph,however, reveals a change in magnitude, whereas the line graph is used to show achange in direction.In summary, line graphs
  • • show specific values of data well • reveal trends and relationships between data • compare trends in different groups of a variableGraphs can give a distorted image of the data. If inconsistent scales on the axes of aline graph force data to appear in a certain way, then a graph can even reveal a trendthat is entirely different from the one intended. This means that the intervals betweenadjacent points along the axis may be dissimilar, or that the same data charted in twographs using different scales will appear different.Example 1 – Plotting a trend over timeFigure 1 shows one obvious trend, the fluctuation in the labour force from January toJuly. The number of students at Andrews high school who are members of the labourforce is scaled using intervals on the y-axis, while the time variable is plotted on the x-axis.The number of students participating in the labour force was 252 in January, 252 inFebruary, 255 in March, 256 in April, 282 in May, 290 in June and 319 in July. Whenexamined further, the graph indicates that the labour force participation of thesestudents was at a plateau for the first four months covered by the graph (January toApril), and for the next three months (May to July) the number increased steadily.Example 2 – Comparing two related variablesFigure 2 is a single line graph comparing two items; in this instance, time is not afactor. The graph compares the number of dollars donated by the age of the donors.According to the trend in the graph, the older the donor, the more money he or shedonates. The 17-year-old donors donate, on average, $84. For the 19-year-olds, theaverage donation increased by $26 to make the average donation of that age group$110.
  • Example 3 – Using correct scaleWhen drawing a line, it is important that you use the correct scale. Otherwise, the linesshape can give readers the wrong impression about the data. Compare Figure 3 withFigure 4:
  • Using a scale of 350 to 430 (Figure 3) focuses on a small range of values. It does notaccurately depict the trend in guilty crime offenders between January and May since itexaggerates that trend and does not relate it to the bigger picture. However, choosing ascale of 0 to 450 (Figure 4) better displays how small the decline in the number of guiltycrime offenders really was.Example 4 – Multiple line graphsA multiple line graph can effectively compare similar items over the same period of time(Figure 5).Figure 5 is an example of a very good graph. The message is clearly stated in the title,and each of the line graphs is properly labelled. It is easy to see from this graph thatthe total cell phone use has been rising steadily since 1996, except for a two-yearperiod (1999 and 2000) where the numbers drop slightly. The pattern of use for womenand men seems to be quite similar with very small discrepancies between them.Scatterplots
  • • Positive or direct relationships • Scattered data points • Data widely spreadIn science, the scatterplot is widely used to present measurements of two or morerelated variables. It is particularly useful when the variables of the y-axis are thought tobe dependent upon the values of the variable of the x-axis (usually an independentvariable).In a scatterplot, the data points are plotted but not joined; the resulting patternindicates the type and strength of the relationship between two or more variables (seeFigure 1 below).Car ownership increases as the household income increases, showing that there is apositive relationship between these two variables.The pattern of the data points on the scatterplot reveals the relationship between thevariables. Scatterplots can illustrate various patterns and relationships, such as: • data correlation • positive or direct relationships between variables • negative or inverse relationships between variables • scattered data points • non-linear patterns • spread of data • outliers.Data correlationWhen the data points form a straight line on the graph, the linear relationship betweenthe variables is stronger and the correlation is higher (Figure 2).
  • Positive or direct relationshipsIf the points cluster around a line that runs from the lower left to upper right of thegraph area, then the relationship between the two variables is positive or direct(Figure 3). An increase in the value of x is more likely associated with an increase in thevalue of y. The closer the points are to the line, the stronger the relationship.Negative or inverse relationshipsIf the points tend to cluster around a line that runs from the upper left to lower right ofthe graph, then the relationship between the two variables is negative or inverse(Figure 4).
  • Scattered data pointsIf the data points are randomly scattered, then there is no relationship between the twovariables; this means there is a low or zero correlation between the variables (Figure 5).Non-linear patternsVery low or zero correlation may result from a non-linear relationship between twovariables. If the relationship is, in fact, non-linear (i.e., points clustering around a curve,not a straight line), the linear correlation coefficient will not be a good measure of thestrength of the relationship (Figure 6).
  • Spread of dataA scatterplot will also illustrate if the data are widely spread or if they are concentratedwithin a smaller area (Figure 7 and 8).
  • OutliersBesides portraying a non-linear relationship between the two variables, a scatterplotcan also show whether or not there exist any outliers in the data (Figure 9).