Designing An Experiment

Biologists often test hypotheses by setting up an experiment. Step Q in Figure 1-9 outlines an experiment to test the hypotheses about how an owl hunts at night. First, experimenters set up a room with an owl perch high on one side and a small trap door on the other side for releasing mice. Then, they tied a leaf to each mouse's tail with a string and released each mouse into the room. Next, each mouse ran silently across the room, but the leaf trailed behind, making a rustling noise. During half of the trials, the lights were on. During the other half, the room was dark. Technicians videotaped all the action in the chamber with an infrared light, which owls cannot see. The researchers then viewed the videos and measured the position of the owl's strike relative to each mouse's head.

Performing the Experiment

Many scientists use a controlled experiment to test their hypotheses. A controlled experiment compares an experimental group and a control group and only has one variable. The control group provides a normal standard against which the biologist can compare results of the experimental group. The experimental group is identical to the control group except for one factor, the independent variable. The experimenter manipulates the independent variable, sometimes called the manipulated variable.

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Predicting Results

Materials 2 Petri dishes with agar, cellophane tape, wax pen

Procedure

1. Open one of the Petri dishes, and streak your finger across the surface of the agar.

2. Replace the lid, and seal it with the tape. Label this Petri dish with your name and a number 1.

3. Seal the second Petri dish without removing the lid. Label this Petri dish with your name and the number 2.

4. Write a prediction about what will happen in each dish. Store your dishes as your teacher directs. Record your observations.

Analysis Was your prediction accurate? What evidence can you cite to support your prediction? If you did not obtain the results you predicted, would you change your testing method or your prediction? Explain. Evaluate the importance of obtaining a result that does not support your prediction.

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