Biology » Introduction to Biology » The Scientific Method

Experimenting and Forming Theories

What is a hypothesis?

Recall from the previous lesson that forming a hypothesis (hi PAHTH us sus) is a research method scientists use often. A hypothesis is an explanation for a question or problem that can be tested. For example, imagine that the number of birds in an area decreased after snakes came into the area. A scientist might make the hypothesis that the snakes were the reason the number of birds decreased.


A scientist who forms a hypothesis must be certain that it can be tested. Before testing a hypothesis, scientists make observations and do research. The results of the experiment will help the scientist answer whether or not the hypothesis is supported.

Experimenting: What is a controlled experiment?

To a scientist, an experiment is a test of a hypothesis by collecting information under controlled conditions.

Controlled experiments involve two groups—the control group and the experimental or test group. The control is the part of an experiment that represents the standard conditions. In other words, the control receives no experimental treatment. The experimental group is the test group that receives experimental treatment.

For instance, imagine an experiment to learn how fertilizer affects plant growth. Fertilizer would be used in the experimental group but not in the control group. All other conditions—soil, light, and water—would be the same for both groups.

In this experiment, using fertilizer is the independent variable. The independent variable is the one condition in an experiment that is tested. How much the plants grow is the dependent variable. The dependent variable is the condition that changes because of a change in the independent variable.

biologist looking at microscope

Safety is another important factor that scientists think about when carrying out investigations and experiments. It is important to know about dangers that may exist from doing an experiment before you begin it. Anyone doing an experiment has a responsibility to follow safety procedures. They must keep themselves and others out of danger. We will look at some lab safety procedures in another lesson.

How are theories formed?

The information gathered from experiments is called data. A scientist carefully reviews or analyzes experimental results to decide if the data supports the hypothesis. Scientists repeat their experiments in order to gather more data. Data are considered reliable only when repeating the experiment several times produces similar results.

Scientists also compare the results of their experiments with the results of other studies. They research published information in scientific journals and computer databases. It is important to have details of an experiment presented in scientific journals and databases so scientists can compare their results with those of similar studies. It lets other scientists test the results by repeating the experiment. If many scientists get the same results, it helps support the hypothesis. A hypothesis that is supported by many different investigations and observations becomes a theory.

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