experimental design. types of variables independent variable (iv) ◦the thing that is intentionally...

Experimental Design

Types of VariablesINDEPENDENT VARIABLE (IV)

◦ The thing that is intentionally CHANGED by the experimenter over the course of the investigation

◦ The manipulated variable

DEPENDENT VARIABLE (DV)

◦ The thing that is MEASURED or OBSERVED by the experimenter

◦ Is affected by the IV

◦ The responding variable

ExampleWater

10” tallOrange Juice

4” tall

Experimental GroupsThe values you choose for your independent variable

Example:If you are studying the effect of amount of water on plant growth, the different amounts of water given to the plants are the experimental groups: 0mL, 5mL, 10 mL, 15 mL

ConstantsFactors in an experiment that are kept the SAME and not allowed to change

Only ONE variable can be tested in a single experiment. All other variables must be held constant. Otherwise an experimenter does not know which independent variable is actually affecting the dependent variable.

Explain examples for a given experiment

Avoid BiasBias: Giving only partial information to get the results that you want

How to avoid bias?◦Repeated Trials◦Larger Sample Size◦Take Accurate Measurements

Imagine this… A weight-loss company makes a new pill that claims to help people lose 3 pounds each week. They do an experiment where they have a person, Jim, take the pills each day for a week.

Jim also recently joined a basketball league and has 2 games that week.

Jim was also sick 2 days that week so didn’t eat for 1 whole day.

At the end of the week, Jim has lost 3 pounds.

The company claims their pill is a success!!

What is wrong with this?

Repeated TrialsExperimental tests done more than onceNecessary to provide more accurate results; data is averaged togetherLessens the impact of human or chance error on the experimental results

ExampleHaving 5 plants for each group (experimental)

HypothesisFormat: If the IV, then the DV, because…

The “because” part of the hypothesis is based on your prior knowledge of scientific concepts

Example: Write a hypothesis for an experiment in which you are testing to see if the drop height of a bouncy ball affects the first bounce height of the bouncy ball

Exit Pass Carter wants to see if the amount of food he feeds a cat affects how many naps the cat takes during a 24 hour period.

Identify the IV, DV, and two constants for this experiment. Write a hypothesis for this experiment.