5 - reliability and validity

5 - reliability and validity - Random and Systematic Error...

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Random and Systematic Error 22:27 Random error: Chance fluctuations in measurement Sources include misreading or misunderstanding of the questions, and  measurement of the individuals on different days or in different places Can occur if the experimenter misprints the questions or misrecords the  answers or if the individual marks the answers incorrectly Random error influences scores on the measured variable in a ways that is self- canceling  These errors will increase the scores of some people and decrease the  scores of other people  The increases and decreases will balance each other and cancel each other  out The measured variable may be influenced by other conceptual variables that are  not part of the conceptual variable of interest  Systematic error: These variables systematically increase or decrease the  scores on the measured variable 
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Reliability 22:27 Reliability: The extent to which a measure is free from random error  One direct way to determine the reliability of a measured variable is to  measure it more than once  Test-Retest Reliability The extent to which scores on the same measured variable correlate with  each other on two different measurements given at two different times o If the measured variable contains random error, the two scores will not be  highly correlated  o Higher positive correlations between the scores at the two times indicate  higher test-retest reliability  Retesting effects: When the same or similar measures are given twice,  responses on the second administration may be influenced by the measure  having been taken the first time  Equivalent-forms reliability: Two different but equivalent versions of the same  measure are given at different times, and the correlation between the scores  on the two versions is assessed  o Useful when there are correct answers to the test that individuals might 
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