Additive Measure

An additive measure is a numeric value in a fact table that can be legitimately summed across any dimension: by customer, by product, by period, by territory.

How it works #

Measures in fact tables fall into three categories:

• Additive: can be summed across all dimensions (e.g., sales amount, quantity, cost). The most common and most useful
• Semi-additive: can be summed across some dimensions but not across time (e.g., account balance: summable by branch, not by month)
• Non-additive: cannot be summed at all (e.g., percentages, ratios, pre-calculated averages)

What it’s for #

Additive measures are the heart of every fact table because they enable the aggregations that the business requires: totals by period, by region, by product. The key rule: always store atomic values (the detail), never aggregates. From a line-level amount you can derive the monthly total; from a monthly total you cannot reconstruct the individual lines.

When to use it #

When designing a fact table, every measure should be classified as additive, semi-additive, or non-additive. This determines which aggregations are valid in reports and which would produce incorrect results. A common mistake is treating a semi-additive measure (like a balance) as if it were additive — summing monthly balances to get a “total” that has no business meaning.