Overview
This lecture explains how to graph categorical data using bar charts and pie charts, and highlights the key differences between bar charts and histograms.
Graphing Categorical Data
- Categorical data uses labels or words, not numbers.
- Two main graphs for categorical data: bar charts and pie charts.
- Bar chart vertical axis represents frequency (count); horizontal axis lists categories (words).
- The biggest distinction: categories on bar charts are labeled with words, not on a number line.
Bar Charts & Pareto Charts
- Bar charts show frequency for each category using rectangles.
- The order of categories on the horizontal axis does not matter for categorical data.
- Pareto chart is a type of bar chart with bars arranged from highest to lowest frequency.
- Bar charts and Pareto charts contain the same data, just ordered differently.
Differences Between Bar Charts and Histograms
- Bar charts are for categorical data; histograms are for numerical data.
- Bar chart axes: categories (words) on horizontal; histogram axes: numbers on a number line.
- Order of bars is flexible in bar charts, fixed in histograms due to numerical order.
- Bar width in bar charts is meaningless (but must be equal); in histograms, width represents numerical intervals.
- Bar charts have gaps between bars to emphasize categories; histogram bars touch to represent continuous data.
Pie Charts
- Pie charts show each category as a slice sized proportional to its frequency/percentage of the total.
- Calculating pie chart slices: frequency of category divided by total number gives percentage.
- Good pie charts always include percentages for clarity.
- Avoid 3D pie charts as they can distort data perception.
Key Terms & Definitions
- Categorical Data — Data described by labels or words rather than numbers.
- Bar Chart — Graph with bars representing the frequency of each category.
- Pareto Chart — Bar chart with bars ordered from highest to lowest frequency.
- Histogram — Graph for numerical data using touching bars to show frequency over numeric intervals.
- Pie Chart — Circular graph where each slice represents a category's proportion of the total.
Action Items / Next Steps
- Review the differences between bar charts and histograms.
- Practice making bar charts and pie charts with categorical data, including labeling percentages.