Know your purpose
- Have a purpose for every table or graph you create
- Choose the table/graph to serve the purpose.
- Pie charts are best to use when you are trying to compare parts of a whole.
- Bar graphs are used to compare things between different groups.
Label your data
- Label your chart to show the categories and indicate whether some have been combined or omitted.
- Name the bars in a bar chart.
- Name the slices in a pie chart.
- If you have omitted some of the cases, make sure the label of the plot defines the collection that is summarized.
Many categories
- A bar chart or pie chart with too many categories might conceal the more important categories.
- In some case, grouping other categories together might be done.
The area principle
- Displays of data must obey a fundamental rule called the area principle.
- The area principle says that the area occupied by a part of the graph should correspond to the amount of data it represents.
- Violations of the area principle are a common way to mislead with statistics.
Misleading graphs: violating area principle
- Decorated graphics: Charts decorated to attract attention often violate the area principle
- Another common violation is when the baseline of a bar chart is not at zero.
- whenever you truncate graph there is a loss in information
- Indicating a y- axis break
- Round-off errors
- Important to check for round-off errors.
- Important to check for round-off errors.
- When table entries are percentages or proportions, the total may sum to a value slightly different from 100% or 1.
- This might result in a pie chart where the total does not add up
Summarizing categorical data
- Graphical summaries of categorical data: bar chart and pie chart.
- Need for a compact measure.
- Numbers that are used to describe data sets are called descriptive measures.
- Descriptive measures that indicate where the center or most typical value of a data set lies are called measures of central tendency.
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