Chapter 10: More on Experiments- Confounding and Obscuring Variables Flashcards
one group, pretest/posttest design?
- one group of participants, measured on a pretest, exposure to treatment, intervention or change and than measured on a posttest. (bad experiment)
What are six threats to internal validity that the one group, pretest/posttest design introduces?
- maturation threats
- history threats
- regression threats
- attrition threats
- testing threats
- instrumentation threats
Maturation threats ?
change in behaviour merges more or press spontaneously over time…people slowly adapt to strange environments.
L> spontaneous remission…symptoms get better for an unknown reason.
Prevention of maturation threats?
comparison group
History threats?
threats to internal validity that occur when a historical or external event occurs to everyone in the treatment group at the same time as the treatment so it is unrelieved ….to be one it must effect everyone or almost everyone
Prevention of History Threats?
a comparison group
Regression threat??
aka regression toward the mean
L> when a performance is extreme at time 1 the next time that performance is measured it is likely to be less extreme than that is closer to a typical of avg performance.
Regress works at??
both extremeness
Regression and internal validity?
when measured in a pretest condition are extreme on DV….most likely to be a threat
L> if a group is unusually high pretest we can expect their scores to go towards the number
Preventing regression?
comparison
Regression is a big threat when?
one group in exactly this situation
Attrition??
occurs when people drop out of the study before it ends.
How does Attrition usually happen in pret-test and post-test designs and when does it become a threat to internal validity?
- when the two tests are administered on separate days…..and its a threat to internal validity when it is systematic…aka when only a certain kind of participant drops out
Is a comparison group a cure all for attrition?
L> if not why?
NO
L> in a two group experiment like a prettest or post-test design if both groups experience the same pattern of dropouts then attrition is not an internal validity threat. But it can be a threat when only one group experiences it.
How do we prevent attrition threats?(2)
- remove the original scores of the participants that drop out so only the scores of those that fully complete the study are included.
- another way is to check the pretest scores of dropouts if they have extreme scores on the pretest their attrition is more threat to internal validity than if it was closer to the group avg.
Testing threat?
L> what type of effect are they?
L> they lead to what five things?
- kind of order effect
- scores have changed over time just because participants have taken the test more than once……they become practiced….bored….jitters for the first testing, sensitization (pretest may sensitize the participant to certain things that influence their post-test answers)
How do we prevent testing threats?
-abandon a pretest all together and use only post test design
- if a pretest is used.they might opt to use alt forms of the test for the two measurements.
- comparison group
L> it takes both the pre test and post test …if a larger effect is seen in the treatment group = testing threats are not harming internal validity.
Instrumentation threats?
- aka instrumental decay
- occur when a measuring instrument changes over time from having been used before.
How do testing threats differ from instrumentation threats?
they occur when a participant changes over time from having been tested before.
L> instrumentation = instruments used to measure
How do we prevent instrumentation threats?
-use only a post-test only design
- if a pre-test, post-test is used….one should make sure the pretest and post-test measurements are equivalent
L> collect data from each instrument and be sure they are calibrated the same
- avoid shifting standards of behavioural coders….via training the coders.. multiple times throughout the experiment establishing reliability and validity at both pretest and post test.
- use clear coding manuals
- counterbalance the versions of the test giving some version A at pretest and version B at post test and giving other participants version B and than version A .
Many threats to one group pretest/post-test design are corrected by adding a comparison group When one has a two group pretest/post-test design what are three more threats to internal validity ?
- observer bias, demand characteristics and placebo effects
Observer bias?
when researchers expectations influence their interpretation of the results or even influence the outcome of the study.
Observer biases can threaten what two validities?
- internal validity because an alt explanation exists for the results….and construct validity of the DV because it means that the results do not represent the true levels
Comparison groups always or do not always control for observer bias?
- not
- if the experimenter knows what participants are in each group their expectations could lead them to see more improvement in the therapy groups vs the comparison group
Demand characteristics?
when participants guess the studies purpose and change their behaviours in the expected direction.