The most effective measurement of a vacuum cleaner's cleaning performance is suction power and airflow.
Suction power demonstrates the actual "pulling" power of a vacuum motor. Strong suction power is needed, for example, when a vacuum cleaner is used on thick, plush, carpeting. Strong motor ability is the only way airflow can continue under obstructed conditions. Conversely, vacuums with weak suction power will "choke" when vacuuming plush carpeting.
Without airflow, there is nothing to pull dirt in and carry it away, which is why suction alone is not effective in carpet cleaning. Some vacuum cleaners have high suction power but low airflow and vice versa. The ideal vacuum cleaner offers a balance of strong suction power and an abundance of airflow: suction power to pull air through plush carpet and sufficient airflow to carry dirt away.