Multiple Bar Charts — Fundamental Concepts

Fundamental Concepts

Read the frame first

Identify the categories, series legend and vertical-axis units. Two bars may represent different years, different groups or different measures. A bar labelled 40 on an axis measured in thousands represents 40,000 units, not forty individual units. Check whether the chart uses a zero baseline or a truncated axis.

Compare the requested quantities

Suppose a hypothetical chart shows branch A rising from 80 to 100 and branch B rising from 40 to 60. Both gain 20 units. Their percentage increases are 25% and 50%, respectively, because the starting values differ. The larger final bar does not establish the larger percentage increase.

For a share of the combined total, add the relevant bars before dividing. For a group average, check whether each bar represents an equally sized group or whether weights are required. Averaging two displayed percentages can be misleading when their underlying populations differ.

Preserve comparability

Compare like units and reporting periods. A count and a rate cannot be subtracted merely because their bars stand next to each other. If a graph does not provide exact labels, read values only to the precision supported by its scale.

The bars describe the dataset presented. A rise after a policy change does not establish that the policy caused it; other changes and the comparison design would need examination.