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PSYC*3250 Ch 4.pdf

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PSYC 3250
Deborah Powell

Friday, October 5, 2012 Chapter 4: Basic Concepts in Measurement and Statistics Psychological Measurement Defining Measurement Measurement - the process of assigning numbers to objects in such a way that specific properties of objects are faithfully represented by properties of numbers Psychological Measurement - the process of assigning numbers to persons in such a way that some attributes of the persons being measured are faithfully reflected by some properties of the numbers Individual Differences - the cornerstone of psychological measurement is the assumption that individuals differ - psychological measurement specialists are interested in assigning individuals numbers that will reflect their differences - the assumption of stable individual differences means that people differ in discernible and potentially measurable ways in their behaviour and that those differences persist over a sufficiently long time to make their measurement useful - psychological tests are designed to measure specific attributes of persons, not the whole person - psychological tests only tell us ways in which individuals are similar or different Scales of Measurement Nominal Scales - numbers are used to classify and identify persons - numbers are substituted for names or verbal labels - numbers are arbitrary Ordinal Scales - people are ranked in terms of an attribute being measured - order does not indicate how close they are (first, second, third) - extremely common in psychology and education, is often sufficient for decision-making Interval Scales - measures differences between objects or people - scale indicates the relative sizes of these differences Ratio Scales - concerned with the relative size, weight, speed or density of 2 objects - there is a zero point - as you go higher on the scales, each one possesses all of the useful properties of the scale before it and then adds more - ratio scales are the most useful but are rare in psychology because a meaningful zero point is often difficult to define Evaluating Psychological Tests - one of the central problems in psychological testing is to determine whether a specific test provides an adequate measure of a specific attribute - (how good a job the test does) - the evaluation of psychological tests as measurement instruments centers on 2 related issues: the reliability of the test scores and the validity of inferences that are made about individuals on the basis of test scores Friday, October 5, 2012 Reliability - a reliable test is one that yields consistent scores when a person takes 2 alternate forms of the test or when he or she takes the same test on two or more different occasions - reliability is the first requirement for good measurement Validity - the correctness of the inferences made based on the scores - there are 2 types of inferences that one might make on the basis of test scores: 1. Inferences regarding the attribute being measured - referred to as validity of measurement - the central question in assessing the validity of measurement is whether the test adequately measures what it purports to measure 2. Inferences that will affect decision made about the test taker - a test that
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