A good scientific experiment is always backed by controls. There are 3 types of controls: positive, negative and blank. Controls allow the user to have confidence that sample results are because of a set variable and not due to a random factor. Furthermore, controls can give an indication of signals which are not due to the target or analyte.
In an experimental procedure, the variable is the treatment which is being assessed. Samples are treated and then analysed. The results are recorded and then compared to the controls. The result from the controls can also help to standardise a procedure by comparint the controls of one test with those of another test for the same target or analyte. If the results for the positive, negative and blanks are similar to previous tests, this indicates that the test was carried out in a similar manner as previously.
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, by using an experimental treatment that is already known to produce that effect (and then comparing this to the treatment that is being investigated in the experiment). The positive control confirms validity of the test, and if it is a quantitative test, it also assesses the 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 that there is no effect when there should be no effect, by using an untreated sample (and then comparing this to the treatment that is being investigated in the experiment). The negative control confirms that no confounding variable is responsible for positive results in the test (ie. that positive results are due to the treatment).
It may be necessary, for some tests to carry out a blank control. This is when only the background solutions are tested, for example, only the buffer with no sample and no treatment. This can be useful to calibrate any machines used to measure the target or analyte, or after the test, as a background control for all tests, negative control, positive control and samples.