Transcript Experimental design:

Experimental design:
Why do psychologists perform experiments?
Correlational methods merely identify relationships:
they cannot establish cause and effect.
A correlation between two variables is inherently
ambiguous:
X might cause Y
Y might cause X
X and Y might both be caused by a third variable or
set of variables.
The experimental method is the best way of identifying
causal relationships.
X causes Y (PIN CAUSES MONEY) if:
X occurs before Y (PIN BEFORE MONEY);
Y happens in the presence of X (MONEY APPEARS WHEN
PIN IS ENTERED;
Y does not happen in absence of X. (NO PIN, NO MONEY).
PIN number:
No PIN number:
Experiments enable us to eliminate alternative
explanations:
To establish causality, we use groups that differ
systematically only on one variable (the independent
variable) and measure the effects of this on an outcome
variable (the dependent variable).
Good experimental designs maximise validity:
Internal validity:
Extent to which we can be sure that changes in the
dependent variable are due to changes in the
independent variable.
External validity (ecological validity):
Extent to which we can generalise from our participants
to other groups (e.g. to real-life situations).
Threats to the internal validity of an experiment's results
(e.g. Campbell and Stanley 1969):
Time threats:
History
Maturation
Selection-maturation interaction
Repeated testing
Instrument change
Group threats:
Initial non-equivalence of groups
Regression to the mean
Differential mortality
Control group awareness of its status.
Participant reactivity threats:
Experimenter effects, reactivity, evaluation apprehension.
History:
Extraneous events between pre-test and post-test affect
participant's post-test performance.
Ask participants how often they use condoms.
Administer advice on safe sexual practices.
Media publicises statistics showing STD's are on the
increase.
Ask participants how often they use condoms.
Changes in reported sexual behaviour may be due to
advice, or due to participants' heightened awareness of
dangers of unsafe sex due to media coverage.
Solution:
Add a control group that is not given advice on safe sex.
Maturation:
Participants may change during the course of the study
(e.g. get older, more experienced, fatigued, etc.).
Effects of an educational intervention on reading ability:
Children's reading ability tested at age 6.
Educational treatment administered.
Children's reading ability tested again, at age 9.
Changes in reading ability may be due to reading
program and/or normal developmental changes with age.
Solution:
Add a control group who do not receive the reading
program, and whose reading ability is tested at ages 6
and 9.
Selection-maturation interaction:
Different participant groups have different maturation
rates, that affect how they respond to the experimenter's
manipulations.
Effectiveness of sex education program.
10-year old boys in experimental group.
8-year old boys in control group.
Pretest on knowledge about sex.
Administer sex education program.
Post-test a year later: experimental group know more
about sex.
But - results may be due to maturational differences
(puberty in older group) as well as exposure to program.
Solution:
Ensure groups differ only on one IV (e.g. in this case
match groups for age).
Time threats: repeated testing.
Taking a pre-test may alter the results of the post-test.
Effects of fatigue on emergency braking in a simulator:
Pre-test: measure driver's braking RT to an unexpected
hazard.
Fatigue induction (30 minutes' simulator driving).
Post-test: measure driver's braking RT to an unexpected
hazard.
Pre-test may alert drivers to possibility of unexpected
tests, and hence maintained arousal at higher levels than
otherwise.
Solution:
In studies like this, avoid repeated testing or add a
control group who get only the post-test.
Instrument change:
e.g. experimenter tests all of one group before testing
another, but becomes more practiced/bored/sloppy while
running the study.
Now two systematic differences between conditions:
Intended experiment:
Actual experiment:
Condition A: drug
Condition A: drug + friendly experimenter
Condition B: no drug
Condition B: no drug + bored experimenter
A problem for observational studies (changes in observer's
sophistication affects scoring of behaviours).
Solution:
Highly standardised procedures; random allocation of
participants to conditions; familiarise oneself with
behaviours before formal observations begin.
Selection (initial non-equivalence of groups):
Cohort effects - groups differ on many variables other
than the one of interest (e.g. gender, age).
Study examines gender differences in attitudes to parking
in disabled bays.
"Females" are also old ladies, "males" are also
Stormtroopers. Cannot conclude observed attitude
differences are due solely to gender.
Solution: often difficult to fix. Match on other IV's?
Regression to the mean:
Participants who give very low or very high scores on one
occasion tend to give less extreme scores when tested
again.
e.g. testing the effectiveness of a remedial reading
program:
test children's reading ability;
select the worst children for the reading program;
re-test children - falsely assume that any improvement is
due to the reading program.
Solution:
Select children randomly, not on basis of low scores.
Avoid floor and ceiling effects with scores.
Differential mortality:
What factors account for missing participants?
(Here, only the really motivated patients overcome their
phobia - so benefits are due to treatment plus personality
factors, not just treatment alone).
Solution: often difficult to fix!
Control group problems that stem from social interaction:
Compensatory rivalry:
If the control group are aware it is not receiving the
experimental treatment, they may show compensatory
rivalry ("John Henry effect") - or resentful demoralisation!
Treatment imitation or diffusion:
Control group imitates the experimental group's treatment,
or benefits from information given to the treatment group
and diffused to the control group.
Compensatory equalization of treatments:
Treatment administrators provide control group with some
benefit to compensate them for lacking the experimental
treatment (e.g. supply an alternative educational
treatment).
Reactivity:
Hawthorne Effect:
Workers' productivity increased after manipulations of pay,
light levels and rest breaks - regardless of nature of
changes made. Workers may have been affected by their
awareness of being studied, as much as by experimental
manipulations.
Draper (2006): review. Productivity may have been
affected by
(a) Material factors, as originally studied, e.g. illumination.
(b) Motivation, e.g. changes in rewards, piecework pay.
(c) Learning (practice).
(d) Feedback on performance.
(e) Attention and expectations of observers.
Implication: act of measurement can affect the very thing
being measured.
Experimenter Effects (e.g. Rosenthal 1966, Rosenthal and
Rosnow 1969):
Expectations of experimenters, teachers, doctors and
managers may affect performance.
Pygmalion effect - teachers expectations affected pupils'
IQ.
Placebo effects - doctors' expectations affect drug effects,
including side-effects.
Solution: "double-blind" procedures if possible.
(e.g. neither doctor nor patient know whether the patient
has been assigned to the drug or placebo condition).
Types of experimental design:
1. Quasi-experimental designs:
No control over allocation of subjects to groups, or
timing of manipulations of the independent variable.
(a) “One-group post-test" design:
treatment
measurement
Prone to time effects, and no baseline against
which to measure effects - pretty useless!
(b) One group pre-test/post test design:
measurement
treatment
measurement
Now have a baseline against which to measure effects of
treatment.
Still prone to time effects.
(c) Interrupted time-series design:
measurement
measurement
time
measurement
treatment
measurement
measurement
measurement
Still prone to time effects.
(c) “Static group comparison" design:
group A:
treatment
(experimental gp.)
measurement
group B:
no treatment
(control gp.)
measurement
Subjects are not allocated randomly to groups; therefore
observed differences may be due to pre-existing group
differences.
Conclusion:
Experiments are the best tool for establishing cause and
effect.
A good experimental design ensures that the only
variable that varies is the independent variable chosen
by the experimenter - the effects of alternative
confounding variables are eliminated (or at least
rendered unsystematic by randomisation).