When we design experiment using scientific method, we have to examine at least 2 groups, so called "experiment" and "control". Both should be kept in the same conditions and under the same influence, excluding one single independent factor which is being examined and should be applied only to the experiment group. Control groups are necessary to verify received results in comparison and identify which changes were caused by targeted influence (present only in experiment group) and which depend on other conditions (present in both experiments and control groups). It makes results more reliable. Finally, in some experiments we have to decrease an influence of the human factor (when researcher subconsciously garbles real results according to results he expects to obtain). Therefore both control and experiment groups are marked and researcher reveals which results are shown for each of them only after making all measurements.