Introduction section of articleIntroduction section of article

• Purpose of study and research question(s)

• Problems related to study topic

• Literature review

• Rationale (justification) for the study

• Hypotheses

Experimental Experimental ResearchResearch

ExampleExample

Objective: To find out the impact of sunshine on plants’ growth.

Treatment

Experimental Control

I.V.I.V.

Experimental ResearchExperimental Research

• Researchers manipulate independent variable - 2 levels

• And measure the other (dependent variable)

• Give treatment to participants and observe if it causes changes in behavior

• Compare experimental group (w/ treatment) with a control group (no treatment)

• Can say IV caused change in the DV

Experimental research designExperimental research design

1.Measure dependent variable

2.Control extraneous variables1.Hold constant

2.The only difference between experimental group and control group is the manipulated variable.

1.Treat groups equally except for treatment.

3.Randomize effects across treatments

4.Design to eliminate alternative explanations

• Random Assignment

– A way to assign participants in your sample to the various treatment conditions (groups that will receive different level of the IV)

– Any member of your sample has equal chance of being assigned in any treatment group

Control VariableControl Variable

• The variable that you would like to keep constant in order to clarify the relationship between Dependent Variable and Independent Variable

• ‘Confounding” variable (if not controlled)

Internal ValidityInternal Validity

• Ability of your research design to adequately test your hypothesis

• Showing that variation in I.V. CAUSED the variation in the D.V. in experiment

• In correlational study,

• Showing that changes in value of criterion variable relate solely to changes in value to predictor variable

• Question: Does new teaching method work better than traditional method in intro psych course?

• Method: Teaches class in morning using new method.

• Teaches afternoon class using traditional method.

• Both classes will use same book, tests, etc.

Example -- Design:Example -- Design:

FindingsFindings

• Students exposed to new method have higher grades.

• Concludes: New method better.

• Justified?

• Why or why not?

ConfoundingConfounding

• Whenever 2 or more variables combine in a way that their effects cannot be separated = confounding.

• Thus, the teaching method study as designed lacks internal validity.

Threats to Internal Validity -- Threats to Internal Validity -- 7 Sources of Confounding7 Sources of Confounding

• Campbell & Stanley (1963)

• History– Other events occur that affect results

• Maturation– Effect of age or fatigue

• Testing

Threats to validityThreats to validity continued continued

• Instrumentation– Changes in criteria used by observers or – instrument - (is scale set to 0?)

• Statistical regression– If participants selected cause of extreme

scores, will tend to be closer to average of pop. upon re-measure.

Threats to internal validityThreats to internal validity cont. cont.

• Biased selection of subjects– If participants differ in ways that affect their

scores on DV

• Experimental mortality– People drop out of study due to frustration,

etc. those that remain differ than drop outs.

Completely Completely randomizedrandomized

• Independent groups

• Completely randomized 2-group experimental design– Advantages

• no pretesting or categorizing of participants • adequate to test hypothesis

2-Treatment 2-Treatment Within-SubjectsWithin-Subjects Design Design

• Repeated measures

• Randomly assign participants to different treatment groups,

• Each participant’s performance is measured under Treatment A and again under Treatment B

• Advantages reduce error variance

• Disadvantages - attrition, carryover effects

2-treatment2-treatment within-subjects within-subjects matched matched designdesign

• Repeated measures• When participant characteristic correlates w

Dependent Variable– Assess the participants for the characteristics– Group participants w/ matching characteristics.

• Matched sets of participants distributed at random, one per group.

• Advantages - Effect of error , effect of characteristic distributed evenly across treatment groups.

Carryover effectsCarryover effects

• First treatment alters the behavior observed in next treatment

• Sources– learning– fatigue– habituation - repeated exposure -- reduced

responsiveness– To effect - (breaks between treatments,

change treatment order)