Why is ancova often used in situations that involve nonequivalent control groups? What is a quantitative measure of some preexisting participant characteristic such as body weight, anxiety, motivation, mental ability, or baseline heart rate that may diffe

Why is ancova often used in situations that involve nonequivalent control groups? What is a quantitative measure of some preexisting participant characteristic such as body weight, anxiety, motivation, mental ability, or baseline heart rate that may differ between groups prior to the administration of any interventions or treatments?