# Driver Formula

The result of a driver formula is a driver quantity. The driver quantity represents the number of units involved in a transaction. The driver quantity, in turn, is used to derive a calculated value which depends on whether the behavior table contains a unit value or a total value.

### Unit Value

When a behavior-table row contains a unit value, the cost per transaction (value) is determined by multiplying the number of units (driver quantity) by the unit cost (in the behavior table) of the transaction.

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### Total Value

When a behavior-table row contains a total value, the cost per transaction (value) is determined in the following way:

1. The total number of units for all transactions (picked out by the selection criteria) is calculated by adding the number of units (as determined by the driver formula) for each transaction (picked out by the selection criteria).
2. The cost per unit is calculated by dividing the total value (in the behavior-table row) by the total number of units.
3. The cost for each transaction (value) is calculated by multiplying the cost per unit by the number of units (as determined by the driver formula) for that transaction.

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