4.4 Rules governing the collection and recording of data 71 5. 6. b. in other instances of that data item in the form being entered (e.g., again in a human resources system, a new allowance for an existing employee must be of a type that is not only different from all other existing allowances for that employee but different from all other new allowances being added at the same time); those that require that multiple data items are consistent with each other: these rules include a. those that require that the values contained in two data items in a transaction or record have a particular relationship to each other: for example, the end date of a period of leave must be no earlier than the start date of that period of leave; b. those that require that the values contained in two or more data items form a valid combination: for example, in a postal address, the place name, state, and postal code 6 must be a valid combination; c. those that require that one set of data items be identical to another set: for example, the set of vendors specified in some real estate transactions must be identical to the current set of regis- tered proprietors; d. those that impose a constraint on a set function over the values contained in a set of data items: for example, the sum of the shares held by the proprietors of a parcel of real property must equal 1; temporal data constraints: those that require that a data item representing a particular date or time