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Canada (161,663)
Psychology (9,695)
PSYC37H3 (159)
Chapter 3

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Anthony Ruocco

Chapter 3Norms and Reliability This chapter concerns two basic conceptso Normso Reliability Scores on psychological tests are interpreted by reference to norms that are based on the distribution of scores obtained by a representative sample of examinees First off the initial outcome or score is useless by itself and in order for a test to be meaningful examiners must be able to convert the initial score to some form of a derived score based on comparison to a standardization or norm groupo The vast majority of tests are interpreted by comparing individual results to a norm group performance however criterion referenced tests are an exception discussed subsequentlyA norm group consists of a sample of examinees who are representative of the population for whom the test is intended When a nationwide sampling is collected for standardization whereas the essential objective of test standardization is to determine the distribution of raw scores in the norm group so that the test developer can publish derived scores known as normso Norms come in many varieties for example percentile ranks age equivalents grade equivalents or standard scores o Norms indicate and examinees standing on the test relative to the performance of other persons of the sae age grade sex and so ono Norms may become outmoded in a few year so periodic renorming of tests should be the rule than an exception Raw Scores The most basic level of information provided by a psychological test is the raw scoreo In personality testing the raw score is often the number of questions answered in the keyed direction for a specific scaleRaw score in isolation is absolutely meaninglessA raw score becomes meaningful mainly in relation to norms an independently established frame of reference derived from a standardization sample Norms are empirically established by administering a test to a large and representative sample of persons The vast majority of psychological tests are interpreting by consulting norms these instruments are callednorm referenced tests Criterion referenced tests help determine whether a person can accomplish an objectively defined criterion such as adding pairs of two digit numbers with 97 percent accuracy norms are not essentialThere are different kinds of norms but they share one characteristico Each incorporates a statistical summary of a large body of scoresEssential Statistical Concepts When confronted with a collection of quantitative data the natural human tendency is to summarize condense and organize it into meaningful patternsFrequency Distributions Frequency distributiona simple and useful way of summarizing data is to tabulate a frequency distributiono This is prepared by specifying a small number of usually equalsized class interval and then tallying how many scores fall within each interval The sum for all interval will equal N the total number of scores in the sample A histogram provides graphic representation of the same information contained in the frequency distributionthe height indicates the number of scores occurring within the interval o Horizontal axisportrays the scores grouped into class intervals o Vertical axisdepicts the number of scores falling within each class intervalFrequency polygon is similar to a histogram except that the frequency of the class intervals is represented by single points rather than columnsthen joined by straight line Measures of Central Tendency The mean M or arithmetic average is one such measure of central tendency o We compute the mean by adding all the scores up and dividing by N the number of scoreso Another useful index of central tendency is the median the middlemost score when all the scores have been rankedo The mode is simply the most frequently occurring score If two scores tie for highest frequency of occurrence the distribution is said to be bimodal If a distribution of scores is skewed that is asymmetrical the median is a better index of central tendency than the mean Measures of Variability Two or more distributions of test scores may have the same mean yet differ greatly in the extent of dispersion of the scores about the mean The most commonly used statistical index of variability in a group of scores is the standard deviationThe standard deviation reflects the degree of dispersion in a group of scoreso If the scores are tightly packed around a central value the standard deviation is smallo The extreme case in which all scores are identical the standard deviation is exactly zeroo The scores when it is more spread out the standard deviation becomes larger
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