The Scientifi c Method

Problem Start with an observation, question, or problem. State the problem. What do you want to fi nd out? Research the problem.

Hypothesis Develop a testable statement that tells what you think will happen.

Experiment Determine the dependent and independent variables. Determine what you will use as controls in the experiment. Determine what types of data you will gather and how.

Conclusion Based on your data, was your hypothesis correct? I f it was not correct, can you develop a new hypothesis?

Theory I f this experiment is repeated many times under the similar conditions, then it may lead to a theory. A theory is an idea that is accepted as true because so much evidence supports it.

For his science project, Waldo has decided to test the effects of 3 different fertilizers on corn plants. He will use brands X, Y, and Z He did some research and found that the numbers on a bag of fertilizers

stand for the percentages of nitrogen, phosphorus, and potassium in the fertilizer.

Step 1: The Problem

• What questions might Waldo have that he wants answered by his experiment?

• “Which fertilizer, X,Y, or Z, causes corn plants to grow the fastest?”

• He looks on the internet and finds that corn plants like growing in soil high in phosphorus.

• What additional questions might he ask once he knows this information?

• “Will the fertilizer highest in phosphorus cause the corn plants to grow fastest?”

• He finds out that fertilizer Y has more phosphorus than the others.

Step 2: The Hypothesis

• Waldo has identified the problem and knows which question he would like answered. Now he needs to form a hypothesis.

• The hypothesis is a statement of what he thinks will happen. It is a question written in the form of a statement.

Waldo’s hypothesis…

• If corn plants are treated with fertilizer X, Y, or Z, then the plants that receive fertilizer Y will grow the fastest because fertilizer Y contains more phosphorus than fertilizer X or Z.

• Notice that Waldo used his research about plant nutrients before forming his hypothesis.

• That’s why a hypothesis is sometimes called an educated guess. You have to research the problem first instead of making a blind guess without any background information.

Step 3: Design an Experiment

• An experiment is a set of steps that are performed to collect data. The data can then be used to prove or disprove the hypothesis.

• The experiment must be designed carefully so that the data collected gives meaningful information.

These questions should be considered when setting up an

experiment.

What data should be collected and how?

• The data must be information that can prove or disprove the hypothesis.

• Data can be qualitative (descriptive) or quantitative (numerical).

• There are three birds at the feeder.

• The birds gathered at the feeder are red.

• Waldo wants to determine which fertilizer will cause the fastest growth of corn plants.

• Since his hypothesis is based on the fastest plant growth, Waldo must base his data on plant growth.

• Waldo decides to measure the height of the corn plants in millimeters every two days for three weeks.

How many groups will be used?How many subjects per group

should be used?& What will be the control

group?

• There are two types of groups in an experiment: control and experimental.

• A control group serves as a standard of comparison. It is treated the exact same way as the experimental groups, but it does not receive the element being experimented.

• Experimental groups all receive the same treatment except for the one element being tested.

• Each group should be made up of more than one member. The more in each group the better. This rules out individual differences.

• Waldo needs to decide- How many groups of plants should he use? What will be his control group? How many experimental groups should he have?

• He decides that his control group will be a group of plants that get the same amount of light and water as the other plants and are grown in the same type of soil but will not get any fertilizer.

• The experimental groups will be the groups of plants that get fertilizer.

• One group will get brand X, one group will get brand Y, and one group will get brand Z.

• With one control group and three experimental groups Waldo will have a total of four groups of corn plants.

• He decides to include five plants in each group.

Will there be a placebo used?

• A placebo is an inert substance given to the control group and should have no effect on the control group.

• The purpose of a placebo is to eliminate biases.

• A bias is an unscientific opinion that may affect the outcome of an experiment.

• Placebos are often used in experiments involving people because the attitudes of people can be a factor in the experiment.

• Waldo decides not to use a placebo because his corn plants will not be affected by biases.

What are the important constants?

• In a well-performed experiment, it is extremely important that all the groups are treated exactly the same except for the experimental element.

• All of the other elements that are kept the same are called constants (sometimes controls).

• The important constants in Waldo’s experiment are soil, temperature, sunlight, and water. Another is the number of plants in each group.

• If these factors are not kept the same, it could affect the data.

• He wants to keep all factors affecting plant growth the same except for the fertilizers. That way, any differences in plant growth will be because of the fertilizer.

• Variables are the things that change during an experiment.

• One variable is tested in each experimental group. No more than one at a time.

• The two types of variables that are important in an experiment: independent variables and dependent variables.

• The independent variable is what the experimenter uses to produce results.

• It is also called the manipulated variable.

• In Waldo’s experiment, the factor he is manipulating to get results from each group is the fertilizer.

• The fertilizer is his independent variable.

What is the independent variable?

What is the dependent variable?

• The dependent variable is what is measured in response to the independent variable.

• It is also called the measured variable.

• Waldo measured the height of the corn plants in response to the fertilizer.

• The height of the plants is the dependent variable.

Picking out variables

• Fill in the sentence:

• ________ depends on ________.

Janice tests the effects of sunlight on bean seed

germination.

• What is the independent variable?

• What is the dependent variable?

Martin tests the effects of caffeine on the heart rate of 10

people.

• What is the independent variable?

• What is the dependent variable?