Posing Questions

As a gardener harvested corn in her vegetable garden, she noticed that on one side of the garden the plants produced very few ears of corn. The gardener wondered, Why didn't the plants on one side of the garden produce as much corn?

An experiment may begin when someone like the gardener asks a specific question or wants to solve a particular problem. Sometimes the original question leads directly to an experiment, but often researchers need to restate the problem before they can design an appropriate experiment. The gardener's question about the corn, for example, is too broad to be tested by an experiment, since there are so many possible different answers. To narrow the topic, the gardener might think about several related questions: Were the seeds the same on both sides of the garden? Was the sunlight the same? Is there something different about the soil?

Formulating Hypotheses

In science, a question about an event is answered by developing a possible explanation called a hypothesis. The hypothesis may be developed after long thought and research or come to a scientist “in a flash.” To be useful, a hypothesis must lead to predictions that can be tested.

In this case, the gardener decided to focus on the quality of the soil on each side of her garden. She did some tests and discovered that the soil had a lower pH on the side where the plants did not produce well. That led her to propose this hypothesis: If the pH of the soil is too low, the plants will produce less corn. The next step is to make a prediction based on the hypothesis, for example, If the pH of the soil is increased using lime, the plants will yield more corn. Notice that the prediction suggests the basic idea for an experiment.

Designing Experiments

A carefully designed experiment can test a prediction in a reliable way, ruling out other possible explanations. As scientists plan their experimental procedures, they pay particular attention to the variables that must be controlled and the procedures that must be defined.

The gardener decided to study three groups of plants:

Group 1—20 plants on the side of the garden with a low pH;

Group 2—20 plants on the side of the garden with a low pH, but with lime added; and

Group 3—20 plants on the side of the garden with a high pH.

Controlling Variables

As researchers design an experiment, they identify the variables, factors that can change. Some common variables include mass, volume, time, temperature, light, and the presence or absence of specific materials. An experiment involves three categories of variables. The factor that scientists purposely change is called the manipulated variable. The factor that may change because of the manipulated variable and that scientists want to observe is called the responding variable. And the factors that scientists purposely keep the same are called the controlled variables. Controlling variables helps make researchers confident that the observed changes in the responding variable are due to changes in the manipulated variable.

For the gardener, the manipulated variable is the pH of the soil. The responding variable is the number of ears of corn produced by the plants. Among the variables that must be controlled are the amount of sunlight received each day, the time of year when seeds are planted, and the amount of water the plants receive.