Although conducting a controlled experiment is not the only way to “do” science, it is a big part of what we will focus on this year in class. Throughout the year we will perform a variety of labs ranging from teacher-led to open inquiry. Regardless of whether you are just identifying them or designing an experiment from scratch, having a working understanding of the following terms will help you to be successful.
Example of Experiment
In the 1600’s Francesco Redi designed an experiment to test the theory of spontaneous generation. This was the leading hypothesis at the time for where life comes from and it stated that life could spring forth from non-living material (ie. maggots from meat).Rediobserved that maggots only appeared on meat that was visited by adult flies. To test the spontaneous generation hypothesis, he placed meat in three sets of jars and added a tight-fitting lid to one, cheesecloth to the other, and the third he left open to the environment. After several days, he noticed that the open container had maggots crawling on the meat. The cheesecloth container had maggots on top of the cloth, but not the meat. Finally, the sealed container had no maggots whatsoever.
CONTROL VARIABLE
Variables that are held constant in order to target the effects of the experimental variable
Example: ______
CONTROL GROUP
A group that receives NO experimental treatment; the results from this group are used to compare to those of the experimental group
Example: ______
INDEPENDENT VARIABLE
Also known as the experimental/manipulated variable; this is what you, as the scientist, are studying the impact of in your experiment; there should only be ONE of these that you are studying in any one experiment
Example: ______
EXPERIMENTAL GROUP
A group that receives the experimental treatment
Example: ______
DEPENDENT VARIABLE
Also known as the responding variable; this is what you are measuring in response to the independent/experimental/manipulated variable
Example: ______
BE CAREFUL
Students often confuse the independent/dependent variables from a graph with those from an experiment. They often can be the same, but not in every case!
The following is an example from the Honors Biology entrance exam that you took last spring:
Water Temperature Experiment
Students studied the effect of ice on the temperature of a sample of water. First, they put 500 mL of cold water (at 10°C) into each of four beakers. Next, they measured and recorded the initial temperature of the water in each beaker. Then, they added various amounts of ice as shown in the table below. They continued to measure the temperature over a period of 30 minutes. Their results are shown in the graph below. The temperature of the room during the experiment was 22°C.
Identify the independent and dependent variables within the experiment. DO not look at the graph! Decide based on what you read in the experimental description.
Experimental independent variable: ______
Experimental dependent variable: ______
On a graph, we say that the independent variable goes on the x-axis (horizontal). The independent variable goes on the y-axis (vertical). Identify the independent and dependent variables from the GRAPH above:
Graph independent variable (x-axis): ______
Graph dependent variable (y-axis): ______