Correlation

  • estimate portion of variance that is error variance
  • degree of consistency or agreement between two independently derived sets of scores
  • stated as a correlation coefficient -1.0 to +1.0
  • Example

Pearson's Product-Moment Correlation Coefficient

  • person's position in group and amount of deviation from group mean
  • significance depends on size of sample
  • 10 cases r=.40 not significant
  • 100 cases r=.40 significant
  • more detail

Spearman's Rank-Order Correlation Coefficient


Test-retest Reliability

  • repeat identical test on a second occassion
  • correlation between scores obtained by same person
  • The scores should be the same minus the error variance
  • error variance corresponds to random fluctuations in performance
  • i.e., broken pencil, illness, fatigue...
  • practice effects
  • must state interval, as r decreases with time
  • >6 months = Coefficient of Stability
  • MMPI example


Alternate-Form Reliability:

  • to avoid problems with test-retest
  • use of comparable forms
  • measures "temporal stability"
  • also measures consistency of response to different item samples
  • concept of "item sampling"
    lucky break versus hard test... what extent to scores on the test depend on factors specific to selection of items
  • short interval = measure of relationship between forms
  • long interval = measure of test-retest and alternate forms
  • very time consuming and work intensive
  • more detail

Split-Half Reliability

  • single administration of test - split in half
  • two scores for each person
  • measure of consistency of content sampling
  • odd versus even
  • more detail

Methods of Estimating Internal Consistency

Method used to determine the extent to which all the items on a given test are measuring the same skill (e.g., the test is "consistently" measuring the same skill.)

  • Spearman-Brown Formula.
  • Inter-item Consistency Formula:
    • The degree of correlation between all the items on a scale (e.g., Cronbach's alpha).
  • Tests of Homogeneity:
    • The degree to which a test measures a single factor, or the extent to which items in a scale are unifactorial.
  • Tests of Heterogeneity: The degree to which a test measures different factors.
  • The Kuder-Richardson formulas: Used on tests with dichotomous items.
    • Kuder-Richardson 20 (KR20) - mean of all possible split-half coefficients
  • The Coefficient Alpha: used for tests that contain non-dichotomous items (items that can individually be scored along a range of values, such as attitude polls and essay tests).
    • Most often used for survey scales and objective assessments
    • yields a lower bound estimate of reliability
    • can be negative if inter-item correlations are negative
    • if items are dichotomously scored, coefficient alpha equals KR20 value

Scorer/Inter-Rater Reliability

  • measure of examiner variance
  • objective versus subjective measures
  • high degree of judgement = high chance of variance
  • measure degree of consistency between two or three examiners
  • .80 or better is good
  • can use Pearson's, Spearmen's or Kappa
  • more detail