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Lecture 2

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Michael Ashton

PSYC 2P25 Sept. 17, 2012 Assessing Quality of Measurement Reliability  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 items  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 characteristic 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 Validity  Means evidence that measurement assess the intended characteristic PSYC 2P25 Sept. 17, 2012  3 kinds of validity: o Content validity  Together, items should assess all aspects of characteristic and not any irrelevant characteristics o Criterion validity  Measurement should correlate in appropriate ways with external :criterion” variables  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 interview o Construct validity  Subsumes content validity and both aspects of criterion validity (convergent and discriminant) 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 Situationism  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 PSYC 2P25 Sept. 17, 2012  There is high r with dishonesty averaged across several other situations (Epstein, 1979)  So trait does exist: across many situations, overall level of dishonesty emerges Mischel & Peake (1982)  Studied trait of organization in university students; observed several behaviours  Found low rs between organized behaviours in any two situations, but when organized behaviour is averaged across many situations there is hi
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