Psychometrics 101: How do I know if an assessment is reliable? (Part 2)

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Posted by Greg Pope

In my last post I offered some general information about assessment reliability. Below are some additional specific things to consider.

  • What factors / test characteristics generally influence internal consistency reliability coefficient values?

A.    Item difficulty: Items that are extremely hard or extremely easy affect discrimination and therefore reliability. If a large number of participants do not have time to finish the test this affects item difficulty
B.    Item discrimination: Items that have higher discrimination values will contribute more to the measurement efficacy of the assessment (more discriminating questions = higher reliability). Part of this relates to sound question development, if questions are well crafted and non-ambiguously worded they are more likely to have acceptable discrimination
C.    Construct being measured: If all questions are measuring the same construct (e.g., from the same topic) reliability will be increased
D.    How many participants took the test: With very small numbers of participants the reliability coefficient will be less stable
E.    Composition of people that took the test: If the sample of participants taking an assessment is not representative (e.g., no-one studied!) the reliability will be negatively impacted
F.    How many questions are administered: Generally the more questions administered the higher the reliability (to a point, we can’t have a 10,000 question test!)
G.    Environmental administration factors: Conditions in the testing area such as noise, lighting levels, etc. can cause distraction away from the measurement of what the participants know and can do
H.    Person factors: Test anxiety, fatigue, and other human factors can reduce the accuracy of measurement of what people know and can do

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For more on this subject see the Questionmark White Paper, “Defensible Assessments: What You Need to Know”

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