Scientific Method Uses Experiments and Controls

Good Experiments and Controls Generate Valid Scientific Conclusions

May 26, 2009

Proper experiments and relevant controls enable scientists to reach logical and valid conclusions as they seek to determine if hypotheses are significant or correct.

Experiments are necessary in the scientific method. The experiments are designed in thoughtful and reasonable ways to support, or disprove, the hypothesis. The rationale and thinking for the experiments should be clear to all who evaluate the research.

Controls and Dependent and Independent Variables in Experiments

Controls are important and critical in any experiment. Controls are those parts of the experiment that are regulated, managed and known by the scientist. One control is the independent variable. The other parts of the experiment that may vary or change are the dependent variables. The scientist controls the independent variable and records what happens to the dependent variable.

Illustrated in two photos below are the effects of pH and amount of inoculum (independent variables) on the zone size (dependent variable) of four different antibiotics (double click to enlarge).

Note some of the dramatic differences. Two pH values and two amounts of inoculum are independent variables in the experiment and they influence the dependent (uncontrolled) variable — the zone sizes shown for each of four tested antibiotics.

Experiments are controlled strictly. Often an experiment will have one independent variable and controlled variables that are kept constant and are strictly defined in the Materials and Methods written for each experiment.

In most cases only a single independent variable is desirable. Multiple independent variables in the same experiment create problems for final analysis and interpretation.

A valid control that is used in many pharmaceutical and medical drug experiments is the placebo, i.e. the pill without any drug. Half the patients get the real drug, and half get the placebo or control pill. Often, the placebo is a sugar pill, which resembles the real drug in size, shape and color. If the drug really works, then half the patients receiving it should do significantly (statistically) better than the placebo group.

Placebos as Controls and the Placebo Effect

Sometimes the simple thought of taking a putative wonder pill makes the test subjects feel better and actually do as well or better than the experimental group. This is called the placebo effect . A surprising number of new drugs have not survived medical trials because of lack of significant differences between drugs and the placebos or controls.

Occasionally, harmful effects are detected in some human subject drug studies that mandate stopping the study.

Another study may dramatically save patients who are at risk of dying and, under these circumstances, to withhold treatment from placebo subjects would be unethical and endanger their lives.

Most medical studies fall in between these two extremes and the placebo participants serve as essential controls for good science.

Scientific Method Summary Concepts

It is important to remember that not all hypotheses are valid. Many hypotheses are proven wrong by the very experiments designed to show them as correct. But, good science is always honest science that seeks the truth by a defined methodology that works quite well.

Sources

Alters, S and B. Alters. 2006, Biology. John Wiley & Sons Inc., Hoboken, N.J. 755 pp

The copyright of the article Scientific Method Uses Experiments and Controls in Scientific Inquiry is owned by Donald Reinhardt. Permission to republish Scientific Method Uses Experiments and Controls in print or online must be granted by the author in writing.