Validity, Reliability, & Sampling Psych 231: Research Methods in Psychology

Errors in measurement Reliability Validity If you measure the same thing twice do you get the same values? Validity Does your measure really measure what it is supposed to measure?? reliable valid unreliable invalid reliable invalid

Reliability True score + measurement error A reliable measure will have a small amount of error Multiple “kinds” of reliability Test-retest Internal consistency Inter-rater

Reliability Test-restest reliability Test the same participants more than once Measurement from the same person at two different times Should be consistent across different administrations Reliable Unreliable

Reliability Internal consistency reliability Multiple items testing the same construct Extent to which scores on the items of a measure correlate with each other Cronbach’s alpha (α) Split-half reliability Correlation of score on one half of the measure with the other half (randomly determined)

Reliability Inter-rater reliability At least 2 raters observe behavior Extent to which raters agree in their observations Are the raters consistent? Requires some training in judgment

Validity Does your measure really measure what it is supposed to measure? There are many “kinds” of validity

Many kinds of Validity VALIDITY CONSTRUCT INTERNAL EXTERNAL FACE CRITERION- ORIENTED PREDICTIVE CONVERGENT CONCURRENT DISCRIMINANT

Many kinds of Validity VALIDITY CONSTRUCT INTERNAL EXTERNAL FACE CRITERION- ORIENTED PREDICTIVE CONVERGENT CONCURRENT DISCRIMINANT

Construct Validity Usually requires multiple studies, a large body of evidence that supports the claim that the measure really tests the construct

Face Validity At the surface level, does it look as if the measure is testing the construct? “This guy seems smart to me, and he got a high score on my IQ measure.”

Internal Validity The precision of the results Did the change in the DV result from the changes in the IV or does it come from something else?

Threats to internal validity The precision of the results History – an event happens the experiment Maturation – participants get older (and other changes) Selection – nonrandom selection may lead to biases Mortality – participants drop out or can’t continue Testing – being in the study actually influences how the participants respond

External Validity Are experiments “real life” behavioral situations, or does the process of control put too much limitation on the “way things really work?”

External Validity Variable representativeness Relevant variables for the behavior studied along which the sample may vary Setting representativeness Are the properties of the research setting similar to those outside the lab (Ecological validity) Subject representativeness Characteristics of sample and target population along these relevant variables

Sampling Why do we do we use sampling methods? Typically don’t have the resources to test everybody, so we test a subset

Sampling Everybody that the research is targeted to be about Population Everybody that the research is targeted to be about The subset of the population that actually participates in the research Sample

Sampling Population Sample Sampling to make data collection manageable Inferential statistics used to generalize back Sampling to make data collection manageable Sample

Sampling Why do we do we use sampling methods? Goals of “good” sampling: Maximize Representativeness: To what extent do the characteristics of those in the sample reflect those in the population Reduce Bias: A systematic difference between those in the sample and those in the population

Sampling Methods Probability sampling Non-probability sampling Simple random sampling Systematic sampling Stratified sampling Non-probability sampling Convenience sampling Quota sampling Have some element of random selection Susceptible to biased selection

Simple random sampling Every individual has a equal and independent chance of being selected from the population

Systematic sampling Selecting every nth person

Stratified sampling Step 1: Identify groups (strata) Step 2: randomly select from each group

Convenience sampling Use the participants who are easy to get

Quota sampling Step 1: identify the specific subgroups Step 2: take from each group until desired number of individuals

Next time Read: Chpt 8