Origin
The Type A behaviour pattern was identified by cardiologists Meyer Friedman and Ray Rosenman (1959), who observed a cluster of behaviours — competitiveness, time-urgency, impatience, and hostility — that they hypothesised raised the risk of coronary heart disease, contrasted with the more relaxed "Type B" pattern. It was later operationalised through the structured interview and the self-report Jenkins Activity Survey (Jenkins, Zyzanski & Rosenman, 1979).
What it measures
The pattern is conceived as a behavioural style rather than a stable personality trait: a tendency toward chronic urgency, drive, and competitiveness (Type A) versus an easy-going, less time-pressured orientation (Type B). Most measures yield a continuum rather than two discrete categories.
Psychometric standing and important caveats
Early prospective work (notably the Western Collaborative Group Study) reported associations between Type A and coronary risk, but subsequent and larger studies failed to replicate the global Type A-heart-disease link, and the field shifted toward identifying hostility as the more reliably cardiotoxic component (Matthews, 1988; Myrtek, 2001, meta-analysis). The Type A construct is therefore best presented with this honest qualification: it is a historically important behavioural framework whose original strong claims about heart disease were substantially weakened by later research.