The measurements made usually fall into a bell shaped curve. With the mean being the highest point in the curve. The more repetitions the smoother the curve. It is very common that the control overlaps with the treated samples. It then requires statistics to determine if the two groups are different.
Controls are an extremely important part of experimentation.
There are several types of controls. Positive control - this is a control that should give a standard result. Negative control should not work. Controls are the single most important aspect of biochemical experiments. The proper controls can save weeks of time trying to interpret results that are caused by contamination, bad solutions or dead cells.