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Definition & Measurement “measurement is the beginning of science, … until you can measure something, your knowledge is meager and unsatisfactory” Lord Kelvin Nominal/Conceptual Definition - define concept in terms of other concepts, links concepts without tying them to real world Operational definition - equates definition with measurement, specify procedures/operations to generate the concept.

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EXAMPLE 1. CONCEPTUAL DEFINITION - Length of table is the distance from one end to the other. OPERATIONAL DEFINITION - Place a yardstick along one edge of the table, place additional yardsticks end to end until one extends over the other edge. Read the numeric marking for inches on the final yardstick at the point where it is exactly over the other edge of the table (call this X inches). Count the number of yardsticks you have used (call this N). Compute (N-1) * 36 + X. Repeat this process on the edge perpendicular to this one. The length of the table in inches is the larger of these two numbers.

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EXAMPLE 2. CONCEPTUAL DEFINITION - Age- the number of years that have passed since a person's date of birth. OPERATIONAL DEFINITION - A. Ask an individual for his name, place of birth (county and state), and names of both parents. Go to the county records office for the given county and find the record for the person with the given name. If there are more than one such records, check for names of parents. Identify the date of birth from this record. Subtract the year of birth from 1998. If the month and day of birth has not yet been reached in the current year, subtract 1.

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AGE : Operational Definition B Give the subject a slip of paper with the following question. ENTER YOUR AGE IN YEARS AT YOUR LAST BIRTHDAY ______. The number written in the blank is the person's age.

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Levels of Measurement Nominal - unordered categories, race Ordinal - ordered categories, sm, med, lg Interval - consistent distance between levels Temperature Fahrenheit Ratio - natural zero Temperature Kelvin

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Levels of Measurement

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NOMINAL Middle income if income is between $30K and $50K Not middle if less than $30K or more than $50K. ORDINAL LOW is less than 30K MID if between 30K and 50K HIGH if greater than 50K INTERVAL Income in dollars from Line 17 of 1997 tax return. RATIO Income in dollars + $10,000. Examples of Measurement scales- Income

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Reliability and Validity of Measures RELIABILITY = Absence of random error (noise), refers to repeatability of the measure. Do we get same answer each time we make measurement? Inter-rater, test-retest, split half, parallel forms, Internal consistency-Cronbach’s alpha, KR VALIDITY = Absence of systematic error (bias). Are we measuring what we purport to be measuring? Content validity ( Trochim- Translation validity ) Criterion-related validity: predictive, concurent, convergent, discriminant Construct validity: MTMM

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Validity vs Reliability

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Statistics & measurement level A dichotomous variable (0 or 1) can be treated as interval

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Assessing Reliability Inter-rater reliability Test-retest Alternative forms Internal consistency –Inter-item correlations –Split half –Cronbach's alpha

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Types of Validity Translation validity (face, content) Criterion-related validity – check against other criteria –Predictive validity – can predict –Concurrent validity – distinguish groups –Convergent validity- correlates with similar measures –Discriminant validity – neg correlation with dissimilar measures

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