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Association is transaction-based. A case consists of a transaction such as a market basket or Web session. The collection of items in the transaction is an attribute of the transaction. Other attributes might be the date, time, location, or user ID associated with the transaction.
In transactional data, a collection of items is associated with each case. However, in most cases only a tiny subset of all possible items are present in a given transaction; the items in the market-basket represent only a small fraction of the items available for sale in the store.
When an item is not present in a collection, it may have a null value or it may simply be missing. Many of the items may be missing or null, since many of the items that could be in the collection are probably not present in any individual transaction.