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A Note on the Standard Length of a Test

Author(s):
Myers, Charles T.
Publication Year:
1961
Report Number:
RB-61-03
Source:
ETS Research Bulletin
Document Type:
Report
Page Count:
13
Subject/Key Words:
Item Analysis, Test Construction, Test Length, Test Reliability

Abstract

The "standard length" of a test is shown to be N = u/c = (v-c)1C where N = the number of typical items that would have a test reliability of .50; u = the average "error" variance in the test items; c = the average inter-item covariance which is also considered to be equal to the "true" variance of the items; and v = the average item variance. Various methods of computing the necessary data from the item-analysis coefficients are shown.

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