Variables, categorical data and graphical displays

Learning objectives

Identify the observational units and the variables in a data set, and distinguish a parameter, which summarizes a population, from a statistic, which summarizes a sample, using the correct symbol for each

Classify a variable as categorical or quantitative, and a quantitative variable as discrete or continuous, recognizing that a number used only as a label is categorical

Construct a frequency table and a relative frequency table for one categorical variable, and convert between counts, proportions, percentages and ratios

Construct and interpret bar charts and pie charts for a categorical variable, and describe its distribution by quoting the most and least common categories with their proportions

Compare the distribution of one categorical variable across two or more groups using relative frequencies, and explain why raw counts mislead when the groups differ in size

Justify or criticize a claim using a table or graph of categorical data, including a bar chart made misleading by a vertical axis that does not start at zero