Positive control
Control samples are necessary to ensure that your experiments are working correctly. Controls are always run side by side with the actual experiment.
Positive controls contain the material that you are detecting. They are required to ensure that the experiment is being run properly. If there is no positive control and the result of the experiment is negative, we cannot be sure that the result is truly negative or if the assay actually did not work. A positive control verifies that the negative results are valid.
Further reading: