Experiment an investigator manipulates one or more

factors (independent variables) to observe their effect on some behavior or mental process (the dependent variable)

by random assignment of participants the experiment controls other relevant factors

An experiment involves a set of controlled conditions that aims to confirm a hypothesis

Hypothesis refers to a statement of cause and effect:

“Higher environmental temperatures lead to more aggression”

“Exposure to marijuana increases appetite”

Direct way to test a hypothesis about a cause-effect relationship between factors

Factors are called variables One variable is controlled by the experimenter

e.g., amount of skittles eaten dailyother examples?

The other is observed and measurede.g., the amount of weight gained other examples?

Independent Variable the experimental factor that is manipulated the variable whose effect is being studiedThe manipulated factor in an experiment hypothesized to cause an effect on another variable

Dependent Variable the experimental factor that may change in

response to manipulations of the independent variable

in psychology it is usually a behavior or mental process

the measured facts hypothesized to be affected

To test a hypothesis, an experimenter defines the variables of the hypothesis:Cause: Independent variable (IV)

Marijuana: Plain cigarette versus cigarette containing 5 mg of THC (the active ingredient in marijuana)

Effect: Dependent variable (DV)Appetite: Grams of ice cream consumed in 1

hourThe experimenter manipulates the IV and

measures the DV to test the hypothesis

Experimental Condition – Gets the IV/treatment the condition of an experiment that exposes

participants to the treatment, that is, to one version of the independent variable

Control Condition- Gets nothing (placebo) the condition of an experiment that contrasts with

the experimental treatment serves as a comparison for evaluating the effect of

the treatment Placebo

an inert substance or condition that may be administered instead of a presumed active agent, such as a drug, to see if it triggers the effects believed to characterize the active agent

Controls are important for determining causality

The only difference between the experimental and control groups is the presence or absence of the IV.

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

Randomization helps avoid false results

Biased sample – when the members of a sample differ in a

systematic way from the larger population in which the researcher is interested

Exampleinterested in all voters, but contact by email so

sample is biased - lower economic groups may not have access; elderly may not have email

Interested in all voters, but call home at 1pm Monday –Friday. Why might this create a biased sample?

Experimenter bias refers to expectations that influence subject behavior

Observer-expectancy effectresearcher has expectations that influence

measurements Subject-expectancy effect

subject knows design and tries to produce expected result

How could either or both be prevented?

Blinding minimize expectancy by removing knowledge about

experimental conditionsSingle-blind study - when subjects are kept

uninformed as to the treatment they are receiving

Double-blind study - when both subjects and experimenter are kept uninformed about aspects of the study that could lead to differential expectations

Which would create the stronger study and why?

•Placebo effects represent changes in behavior that are related to expectations of a treatment

–Placebo effects are controlled by a blind control group (the subject does not know whether the treatment was given or not)

Describe (3) sources of bias and (2) ways to eliminate bias in an experiment

- Other influences BEYOND the control of the research that may contribute to the dependent variable effects

- Clothing & Popularity- Hair length & Confidence

Why do psychologists study animals?

Is it ethical to experiment on animals?

Is it ethical to experiment on people?

Respecting the rights of human research participants involves:

Informed consent is an explanation of a study and the responsibilities of experimenter and participant

Deception involving the subjects must be justifiedConfidentiality of study information must be

maintainedDebriefing refers to explaining the research

process to the subjects at the end of the study

Animal research must be justified and must minimize discomfort to participants

Right to privacyInformed consent

use of deceptionAnimal rights

Is there justification for discomfort or harm a research procedure may produce?

APA publishes ethical guidelines