Scientists are often interested in studying the relationship between 2 experimental variables.  For example:

• What relationship, if any, exists between the amount of fiber in a person’s diet and the risk of developing colon cancer?
• What relationship, if any, exists between the amount of exercise a person gets each week and the probability of developing heart disease?
• What relationship, if any, exists between the wavelength of light shined through a solution of KMnO₄, and the amount of light that is absorbed?
• When light of 520 nm is shined through a solution of KMnO₄, what relationship, if any, exists between the concentration of the solution and the amount of light that is absorbed?

To examine some methods scientists use to analyze the relationship between experimental variables, let’s look at an example where a scientist is studying the relationship between the protein concentration of a solution and the amount of UV light at 280 nm that the solution will absorb.  To study this relationship, the scientist prepares 9 solutions with the following protein concentrations in μg/mL: 0, 10, 20, 40, 80, 160, 320, 640, and 1280.  He then places each solution in a spectrophotometer and measures the absorbance of UV light at 280 nm.  He records the following A₂₈₀ values: 0.000, 0.045, 0.098, 0.195, 0.373, 0.711, 1.398, 1.833, and 1.999.

Notice that the data this scientist collects will consist of pairs of values.  For each observation, he records both the concentration of the solution, and the A₂₈₀ value of the solution.  Since both concentration and the A₂₈₀ values can change as he makes his observations, they are both variables.  However, note that before the experiment even starts, the scientist must decide which protein concentrations he wishes to test.  Because the concentration of each solution was determined beforehand by the experimenter and did not depend, in any way, on the A₂₈₀ values of the solutions or on any other variables in the experiment, it is called an independent variable.  On the other hand, the A₂₈₀ values are not determined by the experimenter.  These values depend on the experimental conditions that have been set up.  Therefore, the A₂₈₀ variable is called a dependent variable.  In general, the results you record at the end of the experiment are the values of your dependent variable.

Example #2 - A scientist studies the survival rate of 4 groups of AIDS patients.  One group receives no drug treatment, the second group receives the drug AZT only, the third group receives the drug saquinavir (a protease inhibitor) only, and the fourth group receives both AZT and saquinavir.  The survival rate is recorded once per month over the course of 2 years.

Table 2.1  Absorbance of UV light by protein solutions

IMPORTANT:  When displaying data in a table:

Protein Concentration (μg/mL)

A₂₈₀

2. All measurements listed in the table should include appropriate units.  (Note, however, that absorbance values are unusual in that they are one of the few measurements that lack units.)