Charts and Graphs

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Charts and Graphs

• Dr. David Cornforth
• School of Information Technology and Electrical
  Engineering
• University of New South Wales _at_ ADFA

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Why use graphs?

• To see data more clearly

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Types of charts
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Axes

• The vertical axis is the dependent variable
• The horizontal axis is the independent variable
• We say a graph of Y against X meaning Y is
  dependent on X.
• Mixing them up completely changes the results of
  regression.

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Interpreting graphs what to look for

• Are all the data points similar?
• Does any one stand out (an outlier)?
• Are there any obvious groups (clusters)?
• Is there any obvious trend, or just random
  variation? How can you tell? (Correlation)
• What applications do these graphs have?

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Non-linear relationships
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Non-linear relationships
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Transforming data

• Makes non-linear plots into straight lines
• Helps to identify the kind of relationship

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Correlation

• A measure of how things are related, or connected
• Applies to XY scatter charts
• In Excel, use three routes
• Tools Data Analysis Correlation
• Click on a chart, select Chart Add Trendline
  Options Display R-squared value
• Insert Function CORREL(array, array)
• Get a number between -1 and 1

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Correlation r

• Degree of similarity between matched data
• Useful indicator of relationships
• Never confuse correlation with causation

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Measures of Correlation

• Pearson's product-moment correlation
• Spearman's rank correlation coefficient

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Correlation versus causation

• Variables may be correlated for any one of
  several reasons
• Chance
• Structural similarity
• Indirect causation
• Direct causation

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A Bad Example

• Whats wrong with this graph?

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Abusing graphs

• Joining the dots on a line graph when there is no
  evidence of continuity.
• On a graph of sales for each type of good this is
  obvious.
• On a graph of sales vs. time its not so obvious.
• Claiming a trend from 2 or 3 data points.

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Exercises

• Using these data, decide the best type of graph
  to use. Sketch the graph. What does it tell you?
  How would you label the axes?

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Useful links

• http//www.spreadsheetconverter.com/excel-chart-we
  b.htm
• http//office.microsoft.com/en-us/excel/HA01034607
  1033.aspx
• http//www.andypope.info/charts.htm
• (VBA code)