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This paper formalizes the concept of similarity which has been used on an ad hoc basis by application engineers to provide more flexibility in concurrency control. We show how the usual correctness criteria of concurrency control, namely, final-state, view, and conflict serializability, can be weakened to incorporate similarity. We extend the weakened correctness criteria described previously for real-time applications which may run continually, have concurrent transaction executions, or skip unimportant computations. A semantic approach based on the similarity concept is then taken to propose a sufficient condition for scheduling real-time transactions without locking of data.