The importance of "controls".

A critical component of the results section is the description of what are known as control experiments.

In fact, one could argue that control experiments are what distinguishes science from non-science, quasi-science and pseudoscience

There are two types of control experiments, positive and negative. In a positive control experiment, we ask whether we are doing what we think we are doing.

For example, if we want to test whether a specific antibiotic kills a newly isolated type of microbe, we would first check that our sample of the antibiotic kills microbes we already know are sensitive to it  - this tells us that the sample of antibiotic we have is active. 

In a negative control experiment, we ask whether the phenomenon we observe is dependent upon what we did.


Consider an antibiotic/microbe experiment. Microbes are generally grown in a solution of water plus various nutrients -- known as the growth medium.

Many antibiotics, however, are very insoluble in water.  For convenience it is common to prepare a stock solution of the drug in a solvent in which is highly soluble, typically ethanol or dimethylsulfoxide (DMSO). Both ethanol and DMSO are completely soluble in water.


Here is an example:  We want to test the effectiveness of an antibiotic.  We make up a 1 mM (10-3 M) stock solution of the antibiotic in DMSO. 

To make our working solution, we dilute one part of the stock solution into 999 parts of growth medium


Brush up on scientific notation


  • What is the final concentration of the antibiotic? 
  • What is the final concentration of DMSO in the growth medium?
  • Describe a control experiment that would tell us that it is the antibiotic, rather than the DMSO, that kills the bacteria. 

Here is another example, from the paper by

Rosenberg, Van Camp & Krigas (1965) Inhibition of cell division in Escherichia coli by electrolysis products from a platinum electrode. Nature 205:698-699.

  • Why is measuring the current in the system a positive control? 
  • What was the negative control in the Rosenberg et al experiment?  What did it control for? 

Use Wikipedia | revised 20 November 2010