Chapter 5: Reliability Flashcards
Reliability
Consistency in measurement; the total variance in an observed distribution of test scores equals the sum of the true variance plus the error variance
Reliability Coefficient
Index of reliability; proportiion that indicates the ratio between the true score variance on a test and the total variance
Concept of Reliability
X = T+ E X = Observed score T = True score E = Error
True Score Model
Also true that the magnitude of the presence of a certain psychological trait as measured b a test of that trait will be due to the true amount of that trait and other factors
Variance
Statistic useful in describing sources of a test score variability; useful because it can be broken down into components
True Variance
Variance from true differences
Error Variance
Variance from irrelevant, random sources
Reliability of a Test
The greater the proportion of the total variance attributed to true variance, the more reliable the test
Sources of Error Variance
Test Construction
Administration
Scoring
Interpretation
Item/Content Sampling
Terms that refer to variation among items within a test as well as to variation among items between tests
Challenge in Test Development
Maximize the proportion of the total variance that is true variance and to minimize the proportion of the total variance that is error variance
Factors related to the Test Environment
Room temperature
Level of Lighting
Amount of ventilation and noise
Instrument used to enter responses and even the writing surface on which responses are written
Factors related to Testtaker variables
Pressing emotional problems
Physical discomfort
Lack of sleep
Effects of drugs or medication
Factors related to Examiner-Related Variables
Examiner’s physical appearance and demeanor; presence or absence of an examiner
Scoring and Scoring systems
Technical glitches may contaminate data
Test-Retest Method
Using the same instrument to measure the same thing at two points in time
Test-Restest Reliability
Result of a reliability evaluation; estimate of reliability obtained by correlating pairs of scores from the same people on two different administrations of the same test
Test-Retest Measure
Appropriate when evaluating the reliability of a test that purports to measure something that is relatively stable over time;
Coefficient of Stability
Estimate of test-retest reliability when the interval between testing is greater than six months
Coefficient of Equivalence
Alternate-Forms or Parraled forms coefficient of reliability
Parallel Forms
Exist when for each form of the test, the means and the variances of observed test scores are equal; means of scores obtained on parallel forms correlate equally with the tue score; scores obtained on parallel test correlate equally with other measures
Alternate Forms
Different versions of a test that have been constructed so as to be parallel; designed to be equivalent with respect to variables such as content and level of difficulty
Similarity between obtaining estimates of alternate forms reliability and parallel forms reliability and obtaining an estimate of test-retest reliability
Two test administrations with the same group are required
Test scores may be affected by factors such as motivation, fatigue, or intervening events such as practice, learning or therapy
Item Sampling
Inherent in the computation of an alternate- or parallel-forms reliability coefficient; testtakers may do better or worse on a specific form of the test not as a function of their true ability but simply because of the particular items that were selected for inclusion in the test
Internal Consistency Estimate of Reliability/Estimate of Inter-Item Consistency
Obtaining an estimate of the reliability of a test without developing an alternate form of the test and without having to administer the test twice to the same people
Split-Half Reliability
Obtained by correlating two pairs of scores obtained from equivalent halves of a single test administered once; useful measure of reliability when it is impractical or undersirable to assess reliability with two tests or to administer a test twice
Steps to compute a Coefficient of Split-Half Reliability
Divide the test into equivalent halves.
Calculate a Pearson r between scores on the two halves of the test
Adjust the half-test reliability using the Spearman-Brown formula
To Split a Test
Randomly assign items to one or the other half of the test; assign odd-numbered items to one half of the test and even-numbered items to the other half
Odd-Even Reliability
assign odd-numbered items to one half of the test and even-numbered items to the other half
Mini Parallel Forms
Each half equal to the other in format, stylistic, statistical, and related aspect
Spearman-Brown Formula
Allows a test developer or user to estimate internal consistency and reliability from a correlation of two halves of a test; Specific application to estimate the reliability of a test that is legnthened or shhortened by any number of items; used to determine the number of items needed to attain a desired level of reliability
In adding items to increase test reliability to a desired level
The rule is that new items must be equivalent in content and difficulty so that the longer test still measures what the original test measured
When Internal Consistency Estimates of Reliability are Inappropriate
When measuring the reliability of a heterogeneous test and speed test
Inter-item Consistency
Refers to the degree of correlation among all the items on a scale; calculated from a single administration of a single form on a test; useful in assessing the homogeneity of a test
Homogeniety
Degree to which a test measures a single factor; extent to which items in a scale are unifactorial
Heterogeneity
Degree to which a test measures different factors; composed of items that measure more than one trait
Nature of Homogeneous Test
The more homogeneous a test is, the more inter-item consistency it can be expected to have;
Desirable because it allows relatively straighforward test-score interpretation
Testtakers with the same score on a Homogeneous Test
Have similar abilities in the area tested