reliable

Reliable is an adjective used to describe something or someone that can be depended upon to perform consistently or to live up to expectations. A reliable person is dependable; a reliable device tends to work as intended under stated conditions. The term is widely used in everyday language to express trust, predictability, and consistency across actions, timelines, or information.

In the context of measurement and research, reliability refers to the degree to which an assessment yields stable and consistent results. Methods include test-retest reliability (consistency over time), internal consistency (how well items on a test measure the same construct, often assessed with Cronbach's alpha), and inter-rater reliability (agreement between observers). A measure may be reliable without being valid; that is, it can produce consistent results that do not reflect the true value.

In engineering and information technology, reliability concerns the probability that a system or component performs its required function under stated conditions for a specified period. Methods to improve reliability include redundancy, fault tolerance, quality control, and maintenance planning. Metrics such as mean time between failures (MTBF) and availability quantify reliability in practice. Reliability is often studied together with maintainability and availability to assess overall system performance.

In everyday usage, "reliable source" refers to information from a source judged to be accurate and trustworthy, while "reliable transportation" or "reliable service" indicates consistency and punctuality. While highly reliable performance is desirable, reliability is context-dependent and may vary with conditions, usage, or time.