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Lecture 2

PSYC 2P25 Lecture Notes - Lecture 2: Internal Consistency, Trait Theory, Convergent Validity

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Michael Ashton

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PSYC 2P25 Sept. 17, 2012
Assessing Quality of Measurement
A measurement is reliable if it agrees with other measurements of the same variable
3 kinds of reliability
o Internal consistency reliability
When scores on a measurement are calculated as a sum (or mean) of various
parts (“items”)
Scores should depend strongly on the common element of the items
Indicates the extent to which scores represent the common element of the
How to make measurements have higher internal-consistency reliability:
Include lots of items adding many items together gives better
measurement of their common characteristic
o Any single item has its own specific element but when we
combine items, these specific parts get cancelled out
Include “items” that are correlated with each other – items that
correlate strongly with each other are measuring a common
o If items are uncorrelated with each other, they don’t have a
common characteristic might be measuring several different
characteristics instead
o Interrater reliability
When a characteristic is measured by obtaining ratings made by several persons
Scores on the total (or average rating should depend strongly on the raters’
common judgement
Indicates the extent to which overall scores represent the common element of
the scores given by the various raters
How to get high interrater reliability:
Have many raters (so that one rater’s idiosyncrasies get cancelled out)
Have only raters whose ratings tend to agree (so that there is a strong
common element to their ratings)
o Stability (Test-Retest Reliability)
If variable is supposed to be a lasting characteristic, then measurements taken
on two occasions (e.g. a few weeks apart) should be highly correlated
Test-retest reliability is usually calculated simply as the correlation between
scores on two occasions
Means evidence that measurement assess the intended characteristic
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PSYC 2P25 Sept. 17, 2012
3 kinds of validity:
o Content validity
Together, items should assess all aspects of characteristic and not any irrelevant
o Criterion validity
Measurement should correlate in appropriate ways with external :criterion”
Convergent validity expect high r with relevant criteria (positive for similar,
negative for opposite)
Discriminant validity expect low r with irrelevant criteria
Exs. Think of “convergent” and “discriminant” criteria for scores on a job
o Construct validity
Subsumes content validity and both aspects of criterion validity (convergent and
Evaluating Reliability and Validity
Use a sample that is representative of the intended population, and ideally a large sample too
Reliability shouldn’t be too low, if it is, then validity can’t be high
“good” level of convergent validity depends on the criterion variable – expect stronger positive
r’s for the theoretically more similar variables (and stronger negative r’s for theoretically more
opposite variables)
Personality Traits
People differ in tendencies to behave in conceptually related ways across situations and over
fairly long time
1960s & 70s: some researchers denied existence of personality traits (ex. Mischel, 1968)
Said that people didn’t differ consistently in behaviour tendencies. Instead, situation
determines which people will show more or less of the behaviour
Empirical Studies: Do Traits Exist?
If a personality trait exists, then people should differ consistently in overall (trait-relevant)
behaviour as averaged across many situations
Hartshorne & May (1928)
Studied trait of dishonesty in children; observed several behaviours
Found low rs between dishonest behaviours in any two situations, but dishonest behaviour is
averaged across several situations
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