positive control

Meaning: A positive control receives a manipulation with an established, expected result. If it produces that result, the procedure and the measurement are working and could have detected an effect of the new treatment. If it does not, the design or the measure is suspect. A negative control is the opposite: no treatment or an inert one such as a placebo, where no change is expected. The same logic appears in lab techniques in biology.

On the MCAT: A passage describes a new intervention, such as a memory-training program, tested alongside a no-training group and a group using a well-established technique such as mnemonics. Questions ask why the extra group was included, which group is the negative control, or what a result means. For example, if the positive control shows no improvement, the likely problem is an insensitive measure or a flawed procedure. You cannot conclude that the new program fails.

Common confusion: "Positive" means a known positive result. It does not mean a pleasant treatment, and it has nothing to do with positive reinforcement. A positive control is also not the experimental group. The experimental group gets the treatment being tested, while the positive control gets a treatment whose effect is already established. It validates the method but does not remove confounds. Random assignment does that.