Purchase Size is a panel data measure. It is the average amount of product purchased by one buying household on a single shopping trip. Purchase Size can be calculated in dollars, units, or EQ volume. In Nielsen the fact is called Item Sales per Item Trip. In IRI the fact is Sales per Trip.
If annual purchase size for Brand X is 1.1 units, it means that Brand X buyers purchased an average of 1.1 units each time they bought Brand X during that year.
If the average price for Brand X is $3.00, purchase size can also be expressed as $3.30.
Sales = (Total # of Households x Penetration) x Buying Rate
Buying Rate = Purchase Frequency x Purchase Size