Two different versions of the instrument are created. We would not want their knowledge to have changed between the first and second testing.Д®quivalent-Form (Parallel or Alternate-Form) Method (measures error because of differences in test forms) The subjects probably would have gained additional mathematics skills during the two months and thus would have scored differently the second time they completed the test. If one were investigating the reliability of a test measuring mathematics skills, it would not be wise to wait two months. This may be a couple weeks to a couple months. One should wait long enough so the subjects donвЂ™t remember how they responded the first time they completed the instrument, but not so long that their knowledge of the material being measured has changed. The trick with test-retest reliability is determining how long to wait between the two administrations. If the results are consistent over time, the scores should be similar. The reliability is the correlation between the scores on the two instruments. The same instrument is given twice to the same group of people. Test-Retest Method (stability: measures error because of changes over time) Relationship of Test Forms and Testing Sessions Required for Reliability Procedures Testing Many tests, such as achievement tests, strive for. 70 indicates 70% consistency in the scores that are produced by the instrument. Some researchers feel that it should be higher. For research purposes, a minimum reliability of. Generally speaking, the longer a test is, the more reliable it tends to be (up to a point). Scorer agreement is consistency of rating a performance or product among different judges who are rating the performance or product. A fourth category (scorer agreement) is often used with performance and product assessments. Internal-consistency measures consistency within the instrument (consistency among the questions). Equivalent-form measures consistency between two versions of an instrument. Test-retest measures consistency from one time to the next. Each measures consistency a bit differently and a given instrument need not meet the requirements of each. There are three major categories of reliability for most instruments: test-retest, equivalent form, and internal consistency. However, an instrument cannot be valid if it is not reliable. Whether those consistent results are valid is another question. From a research point of view, the scale seems to be reliable because whatever it is measuring, it is measuring it consistently. The scale is producing consistent results. Suppose I were to step off the scale and stand on it again, and again it read 15 pounds. If I were to stand on a scale and the scale read 15 pounds, I might wonder. Reliability (visit the concept map that shows the various types of reliability)ДЄ test is reliable to the extent that whatever it measures, it measures it consistently.

Validity and reliability of instruments