Scientific experiments should always be backed by controls. Controls give the users the confidence that sample results have not been obtained due to a random factor. In any experiment, the samples under investigation must be analysed, but the recorded results must also be compared to those from the control in order to identify the variables that yield those particular results. On the the other hand, the results obtained from control experiments can also help standardise a procedure by comparing the controls from one test with those from another one for the same target. If the results obtained from one of the controls are similar to any previous test, the user can conclude that the test was carried out in a similar manner. There are 3 types of controls: positive, negative and blank.
A positive control is a test in which a positive result is expected (ie. the expected phenomenon will occur). This proves that there is an effect when there should be an effect. For positive controls, an experiment that s known to produce a particular effect is used. Then, the results obtained are compared to those obtained in the experiment that is actually being tested. The positive control confirms validity of the test, and if it is a quantitative test, it also assesses sensitivity. If the positive control does not produce the expected outcome, this will indicate that the test is flawed.
A negative control is a test in which a negative result is expected (ie. the expected phenomenon will not occur). This proves there is no effect when there should be no effect. Untreated samples are normally used as negative results and the results obtained from the experiment under investigation are compared against these. The negative control confirms that no confounding variable is being responsible for positive results in the test (ie. that positive results are due to the treatment employed).
For some experiments, running a blank control may also be necessary. For blank controls, background solutions such as buffers with no sample are normally used. Blank controls are useful to calibrate the equipment used to measure the target or analyte, or after the test as a background control for negative and positives controls and samples.