In Psychological Testing,Discrepancies between ERROR does not imply true ability and that a mistake has measurement of been made. It implies ability constitute that there will always errors of be inaccuracy in measurement. measurements.
Tests that are free of measurement error Tests that have are deemed to be too much error reliable. are deemed to be unreliable.
It is assumed that each person has a The difference between true score that the true score and the would be obtained observed score results if there were no from measurement error. errors in measurement. X -T= E Where X – observed score T- true score E- error
It is assumed that the true score for an individual will Because of random error, however, repeated not change with applications of the repeated same test can produce applications of different scores. the same test.
The standard The standard error of measurements tell us, on deviation will be the average, how much the the standard error score varies from the true of measurement. score. In practice, the standardRemember that the deviation of the observed standard deviation score and the reliability of tells us about the the test are used to estimate the standard average deviation error of measurement. around the mean.
Federal government guidelines require that a test be reliable before one can use it to make employment and educational placement decisions (Heubert and Hauser, 1999).
Models of ReliabilityTime Sampling: The Is used to evaluate the Test -Retest Method error associated with administering a test at 2 different times. Administer the same test on 2 well-specified occassions and find the correlation between scores from the 2 administrations.
Models of ReliabilityItem Sampling: Parallel • Determines the error Forms Method variance that is Equivalent Forms attributable to the Reliability selection of one Parallel Forms particular set of items • Compares two equivalent forms of a test that measure the same attribute • Pearson Product Moment Correlation
Models of Reliability• Split Half • A test is given and is divided into halves Method that are scored separately. The results of one half of the test are then compared with the results of the other. • Odd-even system • Correlation between the 2 halves
• Kuder-Richardson 20 • Use to calculate for the Formula (KR20) reliability of the test in which the items are dichotomous, scored 0 or 1 (usually for right or wrong) • Sum of the product of people passing each item times the proportion of people failing each item
Models of Reliability• Split Half Method • Spearman- Brown Formula: use to correct for the half length of the test
Kuder-Richardson A special case of the reliability formula that 21 (KR21) does not require the calculation of the p’s and q’s., instead it uses the mean test score Assumes that all items are of average difficulty
Coeficient Alpha The most generalCronbach Alpha method of finding estimates of reliability through internal consistency.
How reliable is reliable?What is “high The answer depends on the use of the test. enough”? .70 - .80 are good enough for the purposes of basic research. In CLINICAL SETTINGS, a .90 reliability index may not be good enough; greater than . 95 should be required