Scientific control
Scientific control allows for comparisons of concepts. It is a part of the scientific method
Scientific method
Scientific method refers to a body of techniques for investigating phenomena, acquiring new knowledge, or correcting and integrating previous knowledge. To be termed scientific, a method of inquiry must be based on gathering empirical and measurable evidence subject to specific principles of...

. Scientific control is often used in discussion of natural experiments. For instance, during drug testing, scientists will try to control two groups to keep them as identical and normal as possible, then allow one group to try the drug. Another example might be testing plant fertilizer by giving it to only half the plants in a garden (the plants that receive no fertilizer are the control group, because they are kept normal).

Scientific control needs not be experimental, and experimentation can sometimes be impossible (as in astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

). The important thing is to try and control variables and attributes
Variable and attribute (research)
In science and research, attribute is a characteristic of an object . Attributes are closely related to variables. Variable is a logical set of attributes. Variables can "vary" - for example, be high or low...

 in the data so that the conclusions drawn are valid
In logic, argument is valid if and only if its conclusion is entailed by its premises, a formula is valid if and only if it is true under every interpretation, and an argument form is valid if and only if every argument of that logical form is valid....

. Controls are used to try and avoid confounding variables, although this can be extremely difficult. In plainer words, scientific controls allow an investigator to make a claim like "Two situations were identical until factor X occurred, and so the new outcome was caused by factor X."

Controlled experiments

There are many forms of controlled or designed experiments. A relatively simple one separates research subjects or specimen
A specimen is a portion/quantity of material for use in testing, examination, or study.BiologyA laboratory specimen is an individual animal, part of an animal, a plant, part of a plant, or a microorganism, used as a representative to study the properties of the whole population of that species or...

 into two groups: an experimental group and a control group. The control group is practically identical to the experimental group, although the experimental group is changed according to some key variable of interest, while the control group remains constant during the experiment. Each field develops their own specific, important controls.

Many of the ideas behind scientific control can be illustrated by the field of medicine. In drug
A pharmaceutical drug, also referred to as medicine, medication or medicament, can be loosely defined as any chemical substance intended for use in the medical diagnosis, cure, treatment, or prevention of disease.- Classification :...

 testing, it is important to carefully verify that the supposed effects of the drug are produced only by the drug itself. Physician
A physician is a health care provider who practices the profession of medicine, which is concerned with promoting, maintaining or restoring human health through the study, diagnosis, and treatment of disease, injury and other physical and mental impairments...

s achieve such control in many ways. They often use Blinded experiments (particularly double-blind
A blind or blinded experiment is a scientific experiment where some of the people involved are prevented from knowing certain information that might lead to conscious or subconscious bias on their part, invalidating the results....

 studies), where some of the experimenters are ignorant of certain details in order to avoid unconsciously affecting the experiment's results. In a clinical trial
Clinical trial
Clinical trials are a set of procedures in medical research and drug development that are conducted to allow safety and efficacy data to be collected for health interventions...

, two (statistically) identical groups of patient
A patient is any recipient of healthcare services. The patient is most often ill or injured and in need of treatment by a physician, advanced practice registered nurse, veterinarian, or other health care provider....

s are compared, one of which receives the drug and one of which receives a placebo. Placebo
A placebo is a simulated or otherwise medically ineffectual treatment for a disease or other medical condition intended to deceive the recipient...

s have become especially crucial in light of research demonstrating the power of the mind (e.g. the control group will be given a sugar pill, to make sure that the effects of the experiment are not from the actions and expectations that come with taking any pill). The group receiving the placebo would be the control group, while the group receiving the actual drug would be the treatment group. Again, in a double-blind experiment, neither the patients nor the doctor know which group receives the real drug until the end; this serves both to curb bias
Bias is an inclination to present or hold a partial perspective at the expense of alternatives. Bias can come in many forms.-In judgement and decision making:...

 and to isolate the effects of the drug.

In experiments involving a surgical procedure
Surgery is an ancient medical specialty that uses operative manual and instrumental techniques on a patient to investigate and/or treat a pathological condition such as disease or injury, or to help improve bodily function or appearance.An act of performing surgery may be called a surgical...

, a sham operated group
Sham surgery
Sham surgery is a faked operative intervention that omits the step thought to be therapeutically necessary. In controlled studies sham surgery is performed in the control population to assess the effect of the intervention under study by neutralizing the placebo effect and reducing bias...

 is used to ensure that the data reflect the effects of the experiment itself, and are not a consequence of the surgery.

In experiments where crop yield
Crop yield
In agriculture, crop yield is not only a measure of the yield of cereal per unit area of land under cultivation, yield is also the seed generation of the plant itself...

 is affected (e.g. soil fertility), the experiment can be controlled by assigning the treatments to randomly selected plots of land. This mitigates the effect of different soil composition on the yield.

Types of variable

Many experiments are designed to include a negative control and a positive control, which are the simplest forms of controls.


Negative controls are groups where the theory
The English word theory was derived from a technical term in Ancient Greek philosophy. The word theoria, , meant "a looking at, viewing, beholding", and referring to contemplation or speculation, as opposed to action...

 expects no phenomenon. This is similar to the idea of falsifiability
Falsifiability or refutability of an assertion, hypothesis or theory is the logical possibility that it can be contradicted by an observation or the outcome of a physical experiment...

: one way to test a theory is to make sure there is no effect when there should be no effect. To continue with the example of drug testing, a negative control is simply a group that has not been administered the drug. We would say that the control group should show a negative or null
Null result
In science, a null result is a result without the expected content: that is, the proposed result is absent. It is an experimental outcome which does not show an otherwise expected effect. This does not imply a result of zero or nothing, simply a result that does not support the hypothesis...


If both the treatment group and the negative control produce the result, it can be inferred that a confounding variable acted on the experiment.


Positive controls are where a phenomenon is expected, but this situation can be used as the "normal" to test another phenomenon anyway. A control group for drug testing becomes a 'positive' control if they agree to a sinus spray that provokes a flu. This new, 'positive control' flu group could then be used in the standard way: divided into two groups, where one is given a treatment and the other is given a placebo.

Positive controls can also be used to assess test validity
Test validity
Test validity concerns the test and assessment procedures used in psychological and educational testing, and the extent to which these measure what they purport to measure...

. If we want to assess a new test's ability to detect the flu, then we can use positive controls (which often involve another different, and more established test) to make sure that the new test works. That is, the well-established experiment delivers the 'positive result' we expect, and can then be used for other experiments.

For example, in an enzyme assay
Enzyme assay
Enzyme assays are laboratory methods for measuring enzymatic activity. They are vital for the study of enzyme kinetics and enzyme inhibition.-Enzyme units:...

 to measure the amount of an enzyme in a set of extracts, a positive control would be an assay containing some of the purified enzyme, and a negative control would contain no extract. The positive control should give a large amount of enzyme activity, while the negative control should give very low to no activity. If the positive control does not produce the expected result, there may be something wrong with the experiment procedure and the experiment is repeated. If both controls behave correctly, the results of the experiment are often concluded to be the effect of the desired variable.

Necessity of controls

Controls are needed to eliminate alternate explanations of experimental results. For example, suppose a researcher feeds an experimental artificial sweetener to sixty laboratory rats
RATS may refer to:* RATS , Regression Analysis of Time Series, a statistical package* Rough Auditing Tool for Security, a computer program...

 and observes that ten of them subsequently die. The underlying cause of death could be the sweetener itself or something unrelated. Other variables, many of which may not be readily obvious, may interfere with the experimental design. For instance, perhaps the rats were simply not supplied with enough food or water, or the water was contaminated and undrinkable, or the rats were under some psychological or physiological stress, etc. Eliminating each of these possible explanations individually would be time-consuming and difficult. Instead, the researcher can use an experimental control, separating the rats into two groups: one group that receives the sweetener and one that does not. The two groups are kept in otherwise identical conditions, and both groups are observed in the same ways. Now, any difference in morbidity between the two groups can be ascribed to the sweetener itself—and no other factor—with much greater confidence.

See also

  • False positive
  • False negative
  • Experiment
    An experiment is a methodical procedure carried out with the goal of verifying, falsifying, or establishing the validity of a hypothesis. Experiments vary greatly in their goal and scale, but always rely on repeatable procedure and logical analysis of the results...

  • Controlling for a variable
    Controlling for a variable
    Controlling for a variable refers to the deliberate varying of the experimental conditions in order to see the impact of a specific variable when predicting the outcome variable . Controlling tends to reduce the experimental error...

  • James Lind carried out what is thought to be the first controlled experiment.
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