Positive control in an experiment is a control group that researchers use to show that an experiment is working as intended by using conditions that guarantee a positive result. Control groups are not required for all experiments and are typically used in experiments with complex conditions.
Positive control groups allow researchers to show that positive results are possible in an experiment if the unknown variable is unable to influence the results. Experiments in which the positive control fails suggest the design of the experiment is flawed. A negative control group uses a control group that is expected to produce negative results, allowing researchers to prove the variable is having an effect on the experimental group.