Experiments Explanatory research True experiments Experimental designs Quasi-experimental designs Internal validity External validity Ethical issues Strengths and weaknesses

Explanatory Research Purpose: to explain, to determine cause/effect What is explained? Variation in the dependent variable What can be studied in an experiment? Limited, narrow causal relationships Variables that can be studied in lab Topics for which theory has been developed

True experiment includes Two groups (experimental and control) Random assignment to groups Variation in independent variable (manipulated by researcher) Measurement of dependent variable

The groups Experimental group – is exposed to independent variable (I.V.) Control group - is not exposed to I.V. I.V. is the only difference between the groups Any differences in dependent variable (D.V.) must be due to I.V.

Assignment to groups Randomization Easy to carry out Can control for unmeasured or uncontrolled factors Matching Specific characteristics matched in both groups May be very precise Requires knowledge of relevant characteristics May not control for omitted factors

Pretesting Measures D.V. before experiment Establishes comparability of experimental and control groups Provides baseline for comparison with posttest May teach or “clue in” subjects (pretest effect) Costs extra

Experimental Designs Classic Pretest-Posttest-Control-Group Groups Pretest I.V. Uncontrolled factors Posttest Change Exper. O1 X O3 O3-O1 Control O2 O4 O4-O2 Effect of I.V. = (O3-O1) – (O4-O2)

Experimental Designs Posttest-Only Effect of I.V. = (O1-O2) Groups Pretest I.V. Uncontrolled factors Posttest Change Exper. N/A X O1 Control O2 Effect of I.V. = (O1-O2) Eliminates effect of pretest

Experimental Designs Solomon four-group Groups Pretest I.V. Uncontrolled factors Posttest Change Exper. 1 O1 X O3 O3-O1 Control 1 O2 O4 O4-O2 Exper. 2 O5 Control 2 O6 Effect of I.V. = (O3-O1) – (O4-O2) or (O5-O6) Effect of pretest = (O3-O5) or (O4-O6)

Quasi-Experimental Designs May be used when true experiment isn’t possible Usually involve fewer controls No control group Approximately equivalent control group May take place in the field May be “ex post facto:” designed after the “treatment”

Internal Validity Source of Invalidity Solution History – outside events Control group Maturation – changes in subjects Testing – subject may learn Instrumentation - measurement Statistical regression - moderation Selection bias- groups not comparable Randomization Mortality – dropping out Contamination (competition, demoralization) Treatment misidentification (experimenter expectations, placebo effect, Hawthorne effect) Randomization, double blind, process analysis

External Validity Generalization to “real world” Often a problem in experiments 2 main issues Would sample subjects behave same way outside lab? Cross-population generalizability: would findings hold for different groups, times, places?

Ethical Issues Deception (misleading subjects about purpose of experiment) Selective distribution of benefits (also risks, harm)

Experiments’ Strengths and Weaknesses Isolation of cause/effect High internal validity Easy to replicate Best used for explanatory studies (testing of hypotheses) Weaknesses External validity may be low or undetermined Ethical issues High cost per subject