Consistency

Consistency is the quality of being reliable, coherent, and free from contradictions across a set of beliefs, data, actions, or system components. In logic and mathematics, a theory is consistent if it does not derive both a statement and its negation; in such a case no contradiction is entailed. In probability and statistics, a method or estimator is consistent if, as the sample size increases, it converges in probability to the true parameter value or state of nature.

In philosophy and psychology, consistency relates to coherent belief systems and to theories such as cognitive consistency theory, which posits that individuals are motivated to maintain harmony among their beliefs, attitudes, and actions, reducing cognitive dissonance.

In computer science and data management, consistency also describes the integrity of systems. ACID database properties ensure that transactions leave data in a consistent state. In software design and user experience, consistency refers to uniform interfaces, terminology, and behavior across components to reduce cognitive load and avoid surprising users.

Time consistency describes the stability of preferences and plans over time; a time-consistent decision maker does not regret choices retroactively when future information is considered. In ethics, moral consistency or coherence demands that beliefs about right and wrong form a unified system rather than containing obvious contradictions.

In practice, achieving consistency involves rigorous reasoning, validation, testing, and adherence to standards. When consistency fails, systems may produce paradoxes, contradictory data, or misleading conclusions, undermining reliability and trust.