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Chapter 5

Chapter 5.docx

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Connie Boudens

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Chapter 5 Measurement ConceptsReliability of Measures Reliability refers to the consistency or stability of a measure of behaviorex Intelligence or shoe size consistentUnderstanding reliability Any measure that you make can be thought of as comprising two components 1 a true score which is the real score on the variable and 2 measurement error An unreliable measure of intelligence contains considerable measurement error and so does not provide an accurate indication of an individuals true intelligenceUnreliable measures cannot be replicatedA personality measure for example will typically have 10 or more questions called itemsdesigned to assess a trait reliability is increased when the number of items increasesCan assess stability of measures using correlational coefficientscorrelation coefficient is a number that tells us how strongly two variables are related to each othermost common Pearson productmoment correlation coefficient The Pearson correlation coefficient symbolized as r can range from 000 to 100 and 000 to 100 A correlation of 000 tells us that the two variables are not related at allThe closer a correlation is to 100 either 100 or 100 the stronger is the relationshipWhen the correlation coefficient is positive a plus sign there is a positive linear relationshiphigh scores on one variable are associated with high scores on the second variable A negative linear relationship is indicated by a minus signhigh scores on one variable are associated with low scores on the second variableIf measure is reliable two scores should be very similar high positive correlation TestRetest ReliabilityTestretest reliability is assessed by measuring the same individuals at two points in time For example the reliability of a test of intelligence could be assessed by giving the measure to a group of people on one day and again a week laterWe would then have two scores for each person and a correlation coefficient could be calculated to determine the relationship between the first test score and the retest scoreMeasures for reliable at least 80Internal Consistency Reliability It is possible to assess reliability by measuring individuals at only one point in time We can do this because most psychological measures are made up of a number of different questions called items Recall that reliability increases with increasing numbers of itemsInternal consistency reliability is the assessment of reliability using responses at only one point in time Because all items measure the same variable they should yield similar or consistent results One indicator of internal consistency is splithalf reliability this is the correlation of the total score on one half of the test with the total score on the other half The two halves are created by randomly dividing the items into two partsthe corrected reliability is termed the SpearmanBrown splithalf reliability coefficient Another commonly used indicator of reliability based on internal consistency called Cronbachs alpha provides us with the average of all possible splithalf reliability coefficientsSuch itemtotal correlations are very informative because they provide information about each individual item
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