What the exam asks
On the digital AP Statistics exam in Bluebook, you will not be asked to draw a chart by hand, but you must be ready to:
- Calculate counts, proportions, percentages, or ratios from a table, bar chart, or pie chart.
- Describe the distribution of a categorical variable in context by citing specific numerical values.
- Compare the distribution of a single categorical variable across two or more groups (especially when group sample sizes differ).
- Justify a claim about a population or sample using evidence from a tabular or graphical display.
Core ideas
Frequency and relative frequency tables
A categorical variable places observational units into categories or groups. To summarize one categorical variable, we count how many observations fall into each category.
- Frequency (count): The number of observations in a specific category.
- Relative frequency (proportion or percentage): The fraction or percent of the total observations in a category.
Suppose a hypothetical survey of commuters records their primary mode of transit:
| Transit mode | Frequency (count) | Relative frequency (proportion) | Relative frequency (percent) |
|---|---|---|---|
| Car | |||
| Train |
Bar charts and pie charts
Two standard graphs display a single categorical variable:
- Bar chart (bar graph): Displays a bar for each category. The height (or length) of each bar equals the category’s frequency or relative frequency. Bars are separated by spaces to emphasize that categories are distinct, not a continuous number line. Categories can be ordered alphabetically, by size (Pareto chart), or by a natural order (like survey ratings from Strongly Disagree to Strongly Agree).
- Pie chart: Represents the whole group () as a circle, with slices whose central angles and areas are proportional to each category’s relative frequency. A pie chart is only appropriate when the categories are mutually exclusive (no overlap) and exhaustive (sum to of the sample).