1.4 The process of science  (Page 3/25)

Each experiment will have one or more variables and one or more controls. A variable    is any part of the experiment that can vary or change during the experiment. There are three types of variables we will discuss: Independent variable(s) of interest, independent variables not of interest (i.e. controlled variables), and dependent variables. Typically, basic experiments will only have one independent variable of interest (i.e. the factor that is being changed in a deliberate manner to determine if it has an impact on the dependent variable). The dependent variable is the one being measured. For example, if an experiment is designed to test the effects of three different brands of plant fertilizer on plant growth, the independent variable of interest is the brand of fertilizer and the dependent variable is plant growth. It is important to realize that independent variables other than the brand of fertilizer could affect plant growth: soil moisture content, amount of sunlight, temperature, etc. In order to prevent these variables from impacting the results, they are "controlled", meaning they are not allowed to change during the experiment. In other words, plants treated with all three brands of fertilizer should have the same amounts of water and sunlight to prevent these variables from interacting with the independent variable of interest. A control group is a part of the experiment that does not change and provides a baseline of comparison to determine the effect of the independent variable of interest on the dependent variable. Look for the variables and controls in the example that follows. An experiment is conducted to test the hypothesis that phosphate limits the growth of algae in freshwater ponds. A series of artificial ponds are filled with water and half of them are treated by adding phosphate each week, while the other half are treated by adding a salt that is known not to be used by algae. The independent variable of interest here is the phosphate (or lack of phosphate), the experimental or treatment cases are the ponds with added phosphate and the control ponds are those with something inert added, such as the salt. Just adding something is also a control against the possibility that adding extra matter to the pond has an effect. If the treated ponds show lesser growth of algae, then we have found support for our hypothesis. If they do not, then we reject our hypothesis. Be aware that rejecting one hypothesis does not determine whether or not the other hypotheses can be accepted; it simply eliminates one hypothesis that is not valid ( [link] ). Using the scientific method, the hypotheses that are inconsistent with experimental data are rejected. In human drug trials, it is common for the control group to be given a placebo (i.e. "sugar pill") so that individuals both groups (experimental and control) are taking a pill. Otherwise, the act of taking a pill would be a variable that wasn't controlled. It is also common for neither the subjects nor the researchers directly working with them to know which group is receiving the placebo. This feature of experiments, called a double-blind design, is included to prevent any bias from influencing the results.