A while the other group does, she knows

A negative control is part of a well-designed scientific experiment. The negative control group is a group in which no response is expected.

It is the opposite of the positive control, in which a known response is expected.

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Definition of Experimental Design

On the news, we often hear about some new miracle drug that cures one thing or another. But how do doctors test whether or not the cures really work? They use a basic experiment to test if a drug will treat a particular condition and how well the treatment works.

An experiment is an orderly procedure used to test the outcome of a particular set of circumstances. For example, if a doctor wanted to know if the flu shot would prevent the flu, she would give the shot to someone who was exposed to the flu virus and observe if the patient caught the flu bug. But how does she know the flu shot actually prevented the flu? This is where controls come into play.Experiments have two major groups: the test subjects and the control subjects. The test subjects are the individuals that are being used to check what happens when something changes. The test subjects in the flu-virus experiment would be those people with headaches who receive a vaccine.

The control subjects are those individuals who don’t get access to whatever is being tested. They are used to compare the test results. One major type of control is the negative control. A negative control does not receive any test or treatment. They simply get observed in their natural state.

Experiment Example

Continuing with our flu shot experiment, let’s say a doctor decides to test and see how effective the flu shot actually is at preventing the flu. So, she finds a group of people who are exposed to the flu virus and gives them the flu shot.

But wait – how does she know if it was the flu shot that actually prevented the illness? I’ve been exposed to plenty of sick people, and I only get sick some of the time.A good experiment needs something to compare the test results to. So, the doctor goes and finds another group of people who are exposed to the virus and gives them a saline shot – a shot that looks like the flu shot but really isn’t. Then, she observes who gets the flu. If the group that received the flu shot doesn’t get sick, while the other group does, she knows the flu shot had some effect. If both groups get sick or both groups avoid the illness, she knows the flu shot didn’t work.

The group that didn’t get the real shot is called the negative control, because they didn’t get any treatment and a response wasn’t expected.

Purpose of Negative Control

Negative controls are important in experimental design. The negative control makes sure that there isn’t anything strange going on that might be mistaken for a result. What if everyone was already immune to the strain of flu being tested? No one would have gotten sick whether they got the flu shot or not.If there was no negative control, then the doctor would have assumed the shot prevented the illness; however, since she had a negative control – a group that didn’t get any treatment and therefore should have gotten sick – she could see if there was a problem. If the negative control also didn’t get the flu, then the doctor would know that it was something else, something that both the test group and the negative control group got, that actually prevented the illness.

That’s the point of a negative control: to have a group that nothing is supposed to happen to, just to weed out any other factors that might affect the experiment.

Lesson Summary

A negative control is a group in an experiment that does not receive any type of treatment and, therefore, should not show any change during the experiment. It is used to control unknown variables during the experiment and to give the scientist something to compare with the test group.

This is opposed to the test subjects, which are the individuals that are being used to check what happens when something changes.

Negative Control Group

The negative control group is the group that does not receive any treatment
Diagram depicting a negative control group
  • The experimental group that does not receive treatment
  • Opposite of test subjects in an experiment
  • Used to control the unknown variables during an experiment

Learning Outcome

Once you’ve finished this lesson, you should be able to:

  • Define experiment
  • Differentiate between test subjects and control subjects
  • Describe and explain the purpose of the negative control group in an experiment
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