Secondary Dimension

A secondary dimension is a second dimension added to a report so each first-dimension row is split by the second — say entry channel broken down by device.

A report's first dimension answers one question; a secondary dimension opens the next question in the same table without changing the report. Its value is discovering the interaction between two dimensions: maybe organic is good overall but only on desktop, and adding a device dimension reveals it. But a secondary dimension has a cost: the row count multiplies and each cell gets smaller and noisier, so adding a third and fourth dimension quickly reaches a table where each cell holds a few people and nothing generalizes. A health rule is to add a secondary dimension to answer a specific question and remove it after the answer, not leave it on the report permanently. A secondary dimension is kin to segmentation but differs: segmentation separates a group, a secondary dimension keeps all groups side by side in one table.