Experimental Design

Showing Cause & Effect Relationships

Limitations of Experiments

• Often criticized for having little to do with actual behavior because of strict laboratory conditions

• Not always Ethical to create “real life” situations• Natural Experiments – Study natural occurring event to

observe and measure the effects of something you could not create or ethically do in a lab.– Problem is you can’t control variables in a Natural Experiment.

Definitions• Hypothesis—A testable prediction of the outcome of the

experiment or research• Null Hypothesis - the statement that the independent

variable will have no effect on the dependent variable. – Rather than trying to "prove" their hypothesis that something will

happen, social scientists actually try to disprove the null hypothesis – that something will NOT happen

– We assume the null hypothesis is correct (that nothing is going to happen) until we can encounter scientific evidence to reject it.

– Helps to avoid confirmation bias

• Variables—factors that change in ways that can be observed, measured, and verified

• Operational definition—precise description of how the variables will be measured

Operational Definitions

• How the researcher will define and measure the key variables in the experiment.

• In evaluating others’ research, first determine if you agree with the researchers’ operational definitions.

Experimental Group

• The subjects in an experiment who are exposed to the treatment (independent variable)

• Also called the experimental condition

• The group being studied and compared to the control group

Control Group

• Are not exposed to the independent variable

• Results are compared to those of the experimental group

• Also called the control condition

Choosing Your Groupsminimizing confounding variables/individual differences

• Randomly Select a Random Sample—every member of the population being studied should have an equal chance of being selected for the study

• Random Assignment—every subject in the study should have an equal chance of being placed in either the experimental or control group

• Randomly select a random sample then randomly assign that sample to the experimental and control groups.

• Randomization helps avoid false results & bias & accounts for individual differences in people.

Experimental Variables

• Independent variable (IV)– the controlled factor in an experiment – hypothesized to cause an effect on another

variable

• Dependent variable (DV)– the measured facts – hypothesized to be affected

Independent Variable• Causes something to happen• The variable manipulated by the experimenter• The variable which should change the

dependent variable• variable is controlled by the experimenter

Dependent Variable

• The experimental variable which is affected by the independent variable

• The “effect variable”

• The outcome of the experiment

• The variable being observed and measured

Knowing the Difference• Find DV first by asking:

– “What is the researcher measuring or looking for in this study?”

• Next, find IV by asking:– “What do the researchers hope will cause the DV in this study?”

• Verify with an If/Then Statement:– If this (independent variable) THEN this happens (dependent variable).– If my subject drinks an energy drink (Ind. Variable) THEN they should

get a surge in energy (Dep. Variable)

OR • They are testing the effect of (IV) on (DV).• Good Way to Remember: An IV in your arm causes something

to happen (DV)

Potential Problems

Experimental Flaws to Look Out For

Confounding Variables

• Variables, other than the independent variable, which could inadvertently influence the dependent variable

• “Outside factors” that could have caused your results.

• Need to be controlled/eliminated in order to draw a true, cause-effect relationship in the experiment.

• Many confounding variables can be eliminated through random assignment.

Confounding Variables: Environmental Differences

• Any differences in the experiment’s conditions– between the experimental and control groups

• Differences include temperature, lighting, noise levels, distractions, etc.

• Ideally, there should be a minimum of environmental differences between the two groups.

Confounding Variables:Expectation Effects(Participant/Researcher Bias)

• Any changes in an experiment’s results due to the subject or researcher anticipating certain outcomes to the experiment

• Change in DV produced by subject’s expectancy that change should happen

• Researcher favoring one group over another

Sources of Bias

• Demand characteristics—subtle cues or signals by the researcher that communicate type of responses that is expected.– Form of Researcher Bias– Also helps to guard against the Clever Hans Effect

• Hawthorne Effect (participant bias) - refers to a change in behavior of the subject because they have a great deal of attention focused on them. – Usually a spurt or elevation in performance or physical

phenomenon is measured.

Eliminating Bias: Placebo

• A non-active substance or condition administered instead of a drug or active agent

• Given to the control group• Reduces expectancy effects• Ever get a boo boo and have your mom or dad to

kiss it and make it better?• Doctors may use Placebos more than you think (

NBC Report on Placebo 2 min.)• “Nocebo” – Patients when told a drug won’t work

can block it from working.

Eliminating Bias: Single Blind Procedure

• An experimental procedure where the research participants are ignorant (blind) to the expected outcome of the experiment

Eliminating Bias:Double Blind Procedure

• Technique in which neither the experimenter nor participant is aware of the group to which participant is assigned

Experiments: Data Analysis

Are My Results Valid & Reliable?

• Validity – Does the experiment measure and predict what it is supposed to?

• Reliable – If repeated, will we get similar results?

Statistically Significant• Possibility that the differences in results between the

experimental and control groups could have occurred by chance is no more than 5 percent

• Must be at least 95% certain the differences between the groups is due to the independent variable

Experiments: Replication

Replication

• Repeating the experiment to determine if similar results are found

• If so, the research is considered reliable.

• Does Vitamin C really prevent colds?

3 Types of Experiments

Experimental Method

• Play “Water, Water Everywhere” (12:20) Segment #2 from Scientific American Frontiers: Video Collection for Introductory Psychology (2nd edition)– Dousing Rods to find water– An experiment is set up to see if this psychic

phenomenon is true.