PSYB04H3 Lecture Notes - Lecture 9: Spontaneous Remission, Internal Validity, Observer-Expectancy Effect
PSYB04
LEC 9 - MORE ON EXPERIMENTS: CONFOUNDING AND OBSCURING VARIABLES
March 9, 2017
Chapter 11
Threats to Internal Validity : Did the Independent Variable Really Cause the Difference?
● 3 of the most common threats to internal validity : design confounds, selection effects, and order
effects
○ Design confound → there’s an alternative explanation b/c the experiment was poorly
designed
■ Another variable happened to vary systematically along w the intended
independent variable
○ Selection effect → confound exists because the diff independent variable groups have
different types of participants
○ Order Effect → there’s an alternative explanations b/c the outcome might be caused by
the independent variable, but also might be caused by the order in which the levels of the
variable are presented
■ Don’t know whether the independent variable is really having an effect
→ The Really Bad Experiment
● One-group, pretest/posttest design
○ Researcher recruits one group of participants, measures them on a pretest,
exposes them to a treatment, intervention, or change, THEN measures them on
a posttest
→ 6 potential Internal Validity Threats in One-group, Pretest/posttest Designs
● Maturation Threats to Internal Validity
○ Maturation threat: change in behaviour that emerges more or less spontaneously
over time
○ Aka spontaneous remission
○ If treatment groups improved significantly more than the comparison groups did,
each researcher could subtract out the effect of maturation when they interpret
their results
● History Threats to Internal Validity
○ Sometimes threat to internal validity occurs b/c something specific has happened
b/w the pretest and posttest
○ History threats: result from external event that affects most members of the
treatment group at the same time as the treatment
■ Makes it unclear whether the change in the experimental group is
caused by treatment received or by historical factor
○ Must affect everyone or almost everyone in the group
● Regression Threats to Internal Validity
○ Regression threat: refers to statistical concept → regression to the mean
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Lec 9 - more on experiments: confounding and obscuring variables. 3 of the most common threats to internal validity : design confounds, selection effects, and order effects. Design confound there"s an alternative explanation b/c the experiment was poorly designed. Another variable happened to vary systematically along w the intended independent variable. Selection effect confound exists because the diff independent variable groups have different types of participants. Order effect there"s an alternative explanations b/c the outcome might be caused by the independent variable, but also might be caused by the order in which the levels of the variable are presented. Don"t know whether the independent variable is really having an effect. Researcher recruits one group of participants, measures them on a pretest, exposes them to a treatment, intervention, or change, then measures them on a posttest. 6 potential internal validity threats in one-group, pretest/posttest designs. Maturation threat: change in behaviour that emerges more or less spontaneously over time.
