Introduction section of article Purpose of study and research question(s) Problems related to study topic Literature review Rationale (justification) for the study Hypotheses

Experimental Research

Example Objective: To find out the impact of sunshine on plants’ growth. Treatment I.V. I.V. Experimental Control

Experimental 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 design Measure dependent variable Control extraneous variables Hold constant The only difference between experimental group and control group is the manipulated variable. Treat groups equally except for treatment. Randomize effects across treatments 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 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 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

Example -- Design: 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.

Findings Students exposed to new method have higher grades. Concludes: New method better. Justified? Why or why not?

Confounding 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 -- 7 Sources of Confounding Campbell & Stanley (1963) History Other events occur that affect results Maturation Effect of age or fatigue Testing

Threats to validity 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 validity 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 randomized Independent groups Completely randomized 2-group experimental design Advantages no pretesting or categorizing of participants adequate to test hypothesis

2-Treatment Within-Subjects 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-treatment within-subjects matched design 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 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)