Pivot table

A spreadsheet tool that summarises a long table of rows by the categories you choose, adding, counting or averaging a value for each combination without writing a formula. Dragging region into rows, quarter into columns and revenue into values turns 12,000 invoice lines into a grid of totals by region and quarter.

For an analyst it's the fastest way to see the shape of raw data: which customers make up most of the revenue, which months carry the returns, which product lines are growing. Four regions across three quarters give a twelve-cell grid that can be read at a glance, and re-cut by product with one drag.

It has two weaknesses. It doesn't update when the source data changes until it's refreshed. It also summarises whatever the rows contain. Any duplicates or blank categories in the source go straight into the totals. Reconcile the grand total with the source before trusting any cell.

Refresh, then reconcile.