8. Positive and Negative controls PDF

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Controlled experiments An experiment with controls usually separates research subjects into two groups: An experimental group and a control group. The control group is practically identical to the treatment group, except for the single variable of interest whose effect is being tested. Positive and Negative controls Positive controls confirm that the procedure is ‘working’. A positive control is similar to the actual experimental test, but is known from previous experience to give a positive result. It confirms that the basic conditions of the experiment were able to produce a positive result, even if none of the actual experimental samples produce a positive result. Negative controls confirm that the procedure is not showing an unrelated effect. The negative control demonstrates the ‘negative’ or base-line result obtained when a test does not produce a measurable positive result; often the value of the negative control is treated as a "background" value to be subtracted from the test sample results. For example, in an enzyme assay to measure the amount of an enzyme in a set of cellular 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 show a large amount of enzyme activity, while the negative control should give very low to no activity. If both the treatment group and the negative control produce the result, it can be inferred that another variable acted on the experiment and the data are discarded. If the positive control does not produce the 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 concluded to be the effect of the desired variable....