Measurement Experiment - effect of IV on DV

Independent Variable (2 or more levels) MANIPULATED a) situational - features in the environment b) task – type of task performed c) instructional – type of instructions given control vs experimental groups NOT MANIPULATED Subject variable – existing differences of participants - cannot infer causality because cannot manipulate control vs comparison group

Dependent Variable (measured) The usefulness of the experiment depends on what is measured and how well you make the measurements Uses operational definition

Dependent variable defined operationally Construct inferred from measure. memory ; attention; social dominance; anxiety; aggression; work ethic; work load; bonding; helping behavior; hunger

Measurement type Covert –gauges events that cannot be observed directly. Empirical –based on directly observable events Self-reported –based on feelings and perception of subject

Scales of measurement Nominal Categorical data No quantitative information E.g males and females Ordinal Ranked scores Know relative position of scores E.g affiliation ranking

Interval Constant separation between values of scale but no meaningful zero. Know relative difference between scores E.g. IQ, temperature Ratio Meaningful zero point. Know absolute difference between scores. E.g. height, reaction time

Reliability Results are repeatable when measured again No measure is 100% reliable (especially behavioral measure) Measurement = True (hypothetical score) + measurement error reliability most likely if use careful measurement procedure

Test-retest reliability varies due to situational changes sloppy measurement tool assessed by correlation

Internal consistency reliability : questionnaires Measure each person one time but compare multiple answers Split-half reliability : correlates the scores on one half of the test with the other half Cronbach’s alpha : calculates the correlation of each item with every other item – alpha is the average of these correlation coefficients Item-total : correlation of each item to the total score (can assess individual questions too)

Inter-rater reliability The extent to which observers agree Reliability tells us about measurement error but does not indicate if we are accurately measuring the variable of interest.

Validity Are you measuring what you think you are measuring? Validity assumes reliability

Construct validity – is it a valid construct to measure and is the measuring instrument the best – ie adequacy of operational definition Content/Face validity – common sense test does there seem to be a relationship between measure and construct Criterion validity – judged by outcome

How good is the measure? predictive - does it accurately predict future behavior convergent -is it meaningfully related to other measures of same thing concurrent - people in groups known to differ on the construct differ on the measure divergent (discriminant)- score on measure not related to other measure theoretically different

If you have no reliability then your scores vary randomly and you cannot assess the impact of the IV If you have no validity then your conclusions will be wrong.