Scientific control
This article needs additional citations for verification. (August 2011) |
A scientific control is an
Controlled experiments
Controls eliminate alternate explanations of experimental results, especially experimental errors and experimenter bias. Many controls are specific to the type of experiment being performed, as in the molecular markers used in
For example, if a researcher feeds an experimental
The simplest types of control are negative and positive controls, and both are found in many different types of experiments.[2] These two controls, when both are successful, are usually sufficient to eliminate most potential confounding variables: it means that the experiment produces a negative result when a negative result is expected, and a positive result when a positive result is expected. Other controls include vehicle controls, sham controls and comparative controls.[2]
Negative
Where there are only two possible outcomes, e.g. positive or negative, if the treatment group and the negative control both produce a negative result, it can be inferred that the treatment had no effect. If the treatment group and the negative control both produce a positive result, it can be inferred that a
In other examples, outcomes might be measured as lengths, times, percentages, and so forth. In the drug testing example, we could measure the percentage of patients cured. In this case, the treatment is inferred to have no effect when the treatment group and the negative control produce the same results. Some improvement is expected in the placebo group due to the
Positive
Positive controls are often used to assess test validity. For example, to assess a new test's ability to detect a disease (its sensitivity), then we can compare it against a different test that is already known to work. The well-established test is a positive control since we already know that the answer to the question (whether the test works) is yes.
Similarly, in an enzyme assay to measure the amount of an enzyme in a set of extracts, a positive control would be an assay containing a known quantity of the purified enzyme (while a negative control would contain no enzyme). 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 experimental procedure, and the experiment is repeated. For difficult or complicated experiments, the result from the positive control can also help in comparison to previous experimental results. For example, if the well-established disease test was determined to have the same effect as found by previous experimenters, this indicates that the experiment is being performed in the same way that the previous experimenters did.
When possible, multiple positive controls may be used—if there is more than one disease test that is known to be effective, more than one might be tested. Multiple positive controls also allow finer comparisons of the results (calibration, or standardization) if the expected results from the positive controls have different sizes. For example, in the enzyme assay discussed above, a
Randomization
In randomization, the groups that receive different experimental treatments are determined randomly. While this does not ensure that there are no differences between the groups, it ensures that the differences are distributed equally, thus correcting for
For example, in experiments where crop yield 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 variations in soil composition on the yield.
Blind experiments
Blinding is the practice of withholding information that may
During the course of an experiment, a participant becomes
Blinding is an important tool of the scientific method, and is used in many fields of research. In some fields, such as medicine, it is considered essential.[4] In clinical research, a trial that is not blinded trial is called an open trial.
See also
- False positives and false negatives
- Designed experiment
- Controlling for a variable
- James Lind cured scurvy using a controlled experiment that has been described as the first clinical trial.[5][6]
- Wait list control group
References
- ISBN 978-0-7167-7674-1. Retrieved 14 February 2015.
- ^ PMID 12391395. Archived from the original(PDF) on 2010-05-29.
- PMID 24973822.
- ^ "Oxford Centre for Evidence-based Medicine – Levels of Evidence (March 2009)". cebm.net. 11 June 2009. Archived from the original on 26 October 2017. Retrieved 2 May 2018.
- ^ Lind, James. "A Treatise of the Scurvy" (PDF). Archived from the original (PDF) on 2 June 2015.
- ISBN 0-684-87181-5.
External links
- Encyclopædia Britannica. Vol. 7 (11th ed.). 1911. .