COMM 88 Lecture Notes - Lecture 14: Interrupted Time Series, Experiment, Time Series
Comm 88 Lecture 14
May 22, 2018
Pre-experiments and Threats to Internal Validity (cont.)
•Threats to internal validity
•Threats related to pre-testing (or measures over time):
•Mortality
•Reactivity effects:
•Participant’s reaction to being studied, rather than to IV/treatment, influences DV
•Demand characteristics
•Hawthorne effect
•Placebo effect
•So, how to remove/control these threats?
•Conduct a TRUE experiment!
•Random assignment to proper conditions
•Be sure to treat groups equally - all group(s) get equal time, attention, etc.
•Experimenter effect/bias - experimenter’s behavior or attitudes, rather than treatment (IV),
influences DV
•How to control experimenter effects?
•Same thing again (true experiment, etc.) but also
•Use “double blind” procedure
•Automate (or script) the experiment
Quasi-experiments
•No random assignment, but try to make up for that
•Quasi-equivalent control group design
•Y1 X1 Y2 (group 1)
•Y1 X2Y2 (group 2)
•Use pre-test scores to “match” groups before manipulation, also usually match groups on
other aspects
•Time series designs- track many observations over time, before and after a manipulation
•Single-group interrupted time series design
•Y1 Y2 Y3 Y4 X1 Y5 Y6 Y7 Y8 (group 1)
•Improves upon the “one-group pretest-posttest”
•Variation: take treatment away and measure again
•Multiple time series design
