Reliability coefficients are variance estimates, meaning that the coefficient denotes the amount of true score variance. Typically the measurement of reliability is reflected in what is called a reliability coefficient. Cronbach’s alpha can be written as a function of the number of test items and the average inter-correlation among the items. The correlation between one set of observations with the second, then, provides a reliability coefficient. Cronbach's Coefficient Alpha has become the most popular way of reporting estimates of the reliability of psychological measures. Ideally, score reliability should be above 0.80. 2.3. C. Reliability Standards. 4 Difficulty Item Difficulty represents the percentage of students who answered a test item correctly. Reliability coefficients of .6 or .7 and above are considered good for classroom tests, and .9 and above is expected for professionally developed instruments. Test Reliability—Basic Concepts Samuel A. Livingston Educational Testing Service, Princeton, New Jersey. (Internal Technically speaking, Cronbach’s alpha is not a statistical test – it is a coefficient of reliability (or consistency). a reliability coefficient of .70 or higher. The reliability of a test is indicated by the reliability coefficient. A good rule of thumb for reliability is that if the test is going to be used to make decisions about peoples lives (e.g., the test is used as a diagnostic tool that will determine treatment, hospitalization, or promotion) then the minimum acceptable coefficient alpha is .90. This correlation is known as the test-retest-reliability coefficient, or the coefficient of stability. Reliability study designs and corresponding reliability coefficients To estimate test-score reliability, at a minimum one needs at least two observations (scores) on the same set of persons (Tables 2a and 2b). It is denoted by the letter "r," and is expressed as a number ranging between 0 and 1.00, with r = 0 indicating no reliability, and r = 1.00 indicating perfect reliability. January 2018 Corresponding author: S. A. Livingston, E-mail: slivingston@ets.org In reality, all tests have some error, so reliability is never 1.00. No learning effect was found when comparing the results of the second measurement with the first measurement (P>.05). Reliability coefficients range from 1.00 (which is highest) to 0.00 (which is lowest). High reliability coefficients are required for standardized tests because they are administered only once and the score on that one test is used to draw conclusions about each student’s level on the trait of interest. Coefficients in the range 0.80-0.90 are considered to be very good for course and licensure assessments. 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