Controls

Suppose you buy a new pregnancy test and want to make sure it actually works before relying on the result. You would run it on a sample you know is positive (to confirm the test can detect pregnancy) and a sample you know is negative (to confirm the test does not give false alarms). Those two checks are controls. Without them, you have no idea whether the test works at all - a positive result might mean the test shows positive for everything, and a negative result might mean it is broken.

Controls are the backbone of any experiment. They are the reference points that let you interpret your results. The MCAT loves to describe an experiment and then ask which control is missing, which control failed, or what the absence of a control means for the conclusions. Get these straight and those questions become free points.

Positive Control

A positive control is a condition that is known to produce the expected effect. Its purpose is to confirm that the experimental setup is working properly.

If you are testing whether a new antibiotic kills bacteria, your positive control is a plate treated with a known effective antibiotic. If the bacteria on the positive control plate do not die, something is wrong with your experimental procedure - maybe the bacteria are resistant, or the incubation conditions are off. The positive control catches equipment or procedural failures.

Negative Control

A negative control is a condition that is known to produce no effect. Its purpose is to confirm that the experiment is not generating false positives.

In the antibiotic experiment, your negative control is a plate with no antibiotic at all (just the growth medium). If bacteria on the negative control plate die anyway, something in the environment is killing them - contamination, wrong temperature, or a problem with the medium. The negative control catches false positives.

Quick Reference

Placebo and Sham Controls

A placebo is an inactive treatment designed to look identical to the real treatment. In a drug trial, the placebo is typically a sugar pill that matches the real pill in size, shape, and color. The placebo group accounts for the psychological effect of believing you are being treated (the placebo effect).

A sham procedure is the surgical equivalent of a placebo. If the experimental group receives a new knee surgery, the sham group might get an incision and stitches without the actual surgical repair. Sham controls are controversial but powerful because they isolate the effect of the procedure itself from the effect of the overall surgical experience.

A vehicle control is used when the treatment is dissolved in a solvent (the “vehicle”). The vehicle control group receives the solvent alone, without the active compound. This ensures that any effect is due to the drug, not the solvent.

Control Group vs. Experimental Group

The control group does not receive the treatment (or receives a placebo). The experimental group receives the actual treatment being tested. By comparing outcomes between the two groups, the researcher can tell whether the treatment had an effect.

A well-designed experiment keeps everything identical between the control and experimental groups except for the independent variable. Same age range, same measurement procedures, same environmental conditions. The only difference should be the treatment itself.

Why Controls Matter on the MCAT

The most common MCAT question pattern for controls is: “A researcher observes Effect X in the experimental group. Which of the following is a valid criticism of this study?” The answer usually involves a missing or inadequate control. Specifically:

• If there is no negative control, you cannot rule out that the effect happens without the treatment
• If there is no positive control, you cannot confirm the experiment could detect an effect
• If the placebo does not match the treatment in appearance, blinding breaks down