Describes a procedure or algorithm that in principle may be successfully executed in a finite period of time, i.e., a decision procedure. Note that effectiveness does not entail feasibility (i.e., computability in a practical amount of time). For example, there exist classes of problems whose calculation is ensured but requires exponentially more resources than are needed to express the problem itself, e.g., problems in $\text{EXPTIME}$ or $\text{EXPSPACE}$; such problems are considered effective although their computation is not considered feasible.