Lecture 3 Variables & Constants

1. Lecture 3 Variables & Constants

2. Experimental Design • So, you've come up with a reasonable and testable hypothesis, now it's time to design and conduct an experiment to test it. • But, before we can actually design an experiment, we need to consider a few things. • First, we need to make sure we test only one variable at a time! • What is a variable? I'm glad you asked....

3. Introduction To Variables • A variable is anything in an experiment that can change or vary (be different). • A variable can change the outcome of an experiment. • There are three general types of variables: • Test (a/k/a Manipulated or Independent) Variable • Outcome (a/k/a Responding or Dependent) Variable • Constant (a/k/a Controlled) Variables

4. Test Variable • The Test Variable is the thing that is purposely changed by the scientist during the experiment. • It is the factor that is being tested. • Since it is being manipulated (changed), is is sometimes called the manipulated variable. • To identify it, ask yourself ”What is being tested in this experiment?” • The best way to understand this, is to look at a couple of examples.

5. Test Variable - Example 1 • A third grade class wants to study the effects of sunlight on plant growth. They take three identical plants and place one in full sun, one in partial sun, and one in full shade. After two weeks they come back and measure the plants' growth. • What did the scientists change? • Amount of sunlight. • So, what is the Test Variable? • Amount of sunlight.

6. Test Variable – Example 2 • A car company wants to test which grade of gasoline will make their enging run most efficiently. They fill up one car with regular gasoline, one with mid-grade gasoline, and one with premium gasoline. They then measure average fuel economy over a test course. • What did the scientists change? • Grade of gasoline. • So, what is the Test Variable? • Grade of gasoline.

7. Outcome Variable • The Outcome Variable is what may change because of the change in the Test Variable. • It is the variable that is being measured in the experiment. (hint: when doing a lab report, look at your data table if unsure) • Since we expect it to change (or ”respond”), it is sometimes called the Responding Variable.

8. Outcome Variable – Example 1 • In example 1, our third grade students changed the amount of light they gave to their plants. Since that is what they changed, the amount of sunlight was the Independent Variable. • Now ask yourself, what did the students expect to change between the three plants? What did they measure? • Amount of Plant Growth • So, what is Outcome Variable? • Amount of Plant Growth

9. Outcome Variable – Example 2 • In example 2, the car makers changed the grade of gasoline they put in their cars. Since that is what they changed, the grade of gasoline was the Independent Variable. • Now ask yourself, what probably changed between the three cars because of the different types of gas they received? What were the car companies measuring? • Fuel Economy • So, what is the Dependent Variable? • Fuel Economy

10. Cause and Effect • It is helpful to think of Test and Outcome Variables as a ”Cause and Effect” relationship. • The Test Variable (the ”Cause”) makes a change (the ”Effect”) in the Outcome Variable. • For example, the amount of sunlight changes the growth of the plant, and the grade of gas changes the fuel efficiency of the car.

11. Constants • Constants are variables that could effect the outcome of the experiment if they were allowed to change. • Since we want to test only the Test Variable, we have to make sure that all other possible variables stay the same.

12. Constants • In example 1, there are many things that could also affect the growth of a plant. • Type of plant • Amount of water • Size of planter • Etc... • All of these must remain ”constant” in order for us to know that the Test Variable is causing the change in the Outcome Variable.

13. Control Groups • Control Groups are groups to whom no Test Variable is given. • We do not have control groups in every experiment. • We only have them when we need to see what would happen if the Test Variable was not applied at all.

14. Controls • For Example: • If we want to determine whether a medicine helps a cold get better faster, we need to know how long it takes to get rid of the cold without any medicine at all. • So, one group of patients would be given the medicine (the ”test” group) and another would not be given anything (the ”control” group).