Statistics Definitions > Internal Consistency Reliability Internal consistency reliability is a way to gauge how well a test or survey is actually measuring what you want it to measure. A simple example: you want to find out how satisfied your customers are with the level of
customer service they receive at your call center. You send out a survey with three questions designed to measure overall satisfaction. Choices for each question are: Strongly agree/Agree/Neutral/Disagree/Strongly disagree. If the survey has good internal consistency, respondents should answer the
same for each question, i.e. three “agrees” or three “strongly disagrees.” If different answers are given, this is a sign that your questions are poorly worded and are not reliably measuring customer satisfaction. Most researchers prefer to include at least two questions that measure the same thing (the above survey has three). Another example: you give students a math test for number sense and logic. High internal consistency would tell you that the test is
measuring those constructs well. Low internal consistency means that your math test is testing something else (like arithmetic skills) instead of, or in addition to, number sense and logic. In order to test for internal consistency, you should send out the surveys at the same time. Sending the surveys out over different periods of time, while testing, could introduce
confounding variables. An informal way to test for internal consistency is just to compare the answers to see if they all agree with each other. In real life, you will likely get a wide variety of answers, making it difficult to see if internal consistency is good or not. A wide variety of statistical tests are available for internal consistency; one of the most widely
used is Cronbach’s Alpha. Next: Cronbach’s Alpha References: ---------------------------------------------------------------------------
Need help with a homework or test question? With Chegg Study, you can get step-by-step solutions to your questions from an expert in the field. Your first 30 minutes with a Chegg tutor is free! Comments? Need to post a correction? Please Contact Us. Reliability (visit the concept map that shows the various types of reliability) A test is reliable to the extent that whatever it measures, it measures it consistently. If I were to stand on a scale and the scale read 15 pounds, I might wonder. Suppose I were to step off the scale and stand on it again, and again it read 15 pounds. The scale is producing consistent results. From a research point of view, the scale seems to be reliable because whatever it is measuring, it is measuring it consistently. Whether those consistent results are valid is another question. However, an instrument cannot be valid if it is not reliable. There are three major categories of reliability for most instruments: test-retest, equivalent form, and internal consistency. Each measures consistency a bit differently and a given instrument need not meet the requirements of each. Test-retest measures consistency from one time to the next. Equivalent-form measures consistency between two versions of an instrument. Internal-consistency measures consistency within the instrument (consistency among the questions). A fourth category (scorer agreement) is often used with performance and product assessments. Scorer agreement is consistency of rating a performance or product among different judges who are rating the performance or product. Generally speaking, the longer a test is, the more reliable it tends to be (up to a point). For research purposes, a minimum reliability of .70 is required for attitude instruments. Some researchers feel that it should be higher. A reliability of .70 indicates 70% consistency in the scores that are produced by the instrument. Many tests, such as achievement tests, strive for .90 or higher reliabilities. Relationship of Test Forms and Testing Sessions Required for Reliability Procedures
Test-Retest Method (stability: measures error because of changes over time) If one were investigating the reliability of a test measuring mathematics skills, it would not be wise to wait two months. 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. We would not want their knowledge to have changed between the first and second testing. Equivalent-Form (Parallel or Alternate-Form) Method (measures error because of differences in test forms) Internal-Consistency Method (measures error because of idiosyncrasies of the test items) – Split-Half – Kuder-Richardson Formula 20 (K-R 20)
and Kuder-Richardson Formula 21 (K-R 21) – Cronbach’s Alpha I have created an Excel spreadsheet that will calculate Spearman-Brown, KR-20, KR-21, and Cronbach’s alpha. The spreadsheet will handle data for a maximum 1000 subjects with a maximum of 100 responses for each. Scoring Agreement (measures error because
of the scorer) – Interrater Reliability – Percentage Agreement ——— All scores contain error. The error is what lowers an instrument’s reliability. ———- There could be a number of reasons why the reliability estimate for a measure is low. Four common sources of inconsistencies of test scores are listed below: Test Taker — perhaps
the subject is having a bad day ———- Del Siegle, Ph.D. Created 9/24/2002 How do you measure internal consistency of an instrument?Internal consistency is usually measured with Cronbach's alpha, a statistic calculated from the pairwise correlations between items. Internal consistency ranges between negative infinity and one. Coefficient alpha will be negative whenever there is greater within-subject variability than between-subject variability.
How do you measure internal consistency reliability?The internal consistency reliability test provides a measure that each of these particular aptitudes is measured correctly and reliably. One way of testing this is by using a test-retest method, where the same test is administered some after the initial test and the results compared.
What method is used to assess internal consistency of a test?Test-retest reliability measures the consistency of results when you repeat the same test on the same sample at a different point in time.
What is a common measure of internal consistency?Perhaps the most common measure of internal consistency used by researchers in psychology is a statistic called Cronbach's α (the Greek letter alpha). Conceptually, α is the mean of all possible split-half correlations for a set of items. For example, there are 252 ways to split a set of 10 items into two sets of five.
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