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Measurement - PowerPoint PPT Presentation

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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

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Experiment - effect of IV on DV

Independent Variable (2 or more levels)


  • a) situational - features in the environment

  • b) task – type of task performed

  • c) instructional – type of instructions given

  • control vs experimental groups


  • 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


Categorical data

No quantitative information

E.g males and females


Ranked scores

Know relative position of scores

E.g affiliation ranking

Scales of measurement


  • Constant separation between values of scale but no meaningful zero.

  • Know relative difference between scores

  • E.g. IQ, temperature


  • Meaningful zero point.

  • Know absolute difference between scores.

  • E.g. height, reaction time


  • 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.


  • 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.

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