Counting backwards by threes (serial subtractions) is a standard cognitive task used to assess working memory, attention, and mental processing speed in clinical and psychological testing; no formal citation is needed for this basic definition.
The claim describes a standard, widely understood cognitive test procedure (serial sevens or serial threes subtraction) used in mental status examinations and psychological tests to evaluate attention and working memory. Because this is a matter of common procedural definition in clinical and psychological practice rather than a contestable empirical discovery, it falls under COMMON_KNOWLEDGE.