(Martin, 1983). Also, the measures taken to keep computer-based information systems safe from unauthorized access and other harm (IOM, 1991a). 

Third-party payers

Companies that administer health benefit plans, maintain records of eligibility and payment, adjudicate, and pay claims. The first and second parties are the patient and the provider (clinician or institution). (When the health plan is administered by the company, they are called third-party administrators.)

Unique identifier

A code (usually numeric or alphanumeric) that refers to one, and only one, person at any one time, does not change for that person over time, and permits positive (or probable) identification of that individual. The term may apply to codes assigned to data subjects and to practitioners. (See also Universal identifier.)

Universal identifier

A single code used in all health databases to refer to an individual. Such a code would allow linkage among health databases. (See Data linkage.)

Validity

The extent to which data correspond to the actual state of affairs or an instrument that measures what it purports to measure.

Internal validity

The degree to which one can support a causal relationship between treatment and outcome, given the way they are measured by the data.

Construct validity

The degree to which one can generalize from one analysis to broader theories or models.

External validity

The degree to which one can generalize from a finding (of causal relationship) to alternative measures of the treatment and outcome and across different types of individuals, sites of care, and times. (See also Generalizability.)