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Psychometrics 101: Foundational Knowledge for Testing Professionals Steve Saladin, Ph.D. University of Idaho.

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Presentation on theme: "Psychometrics 101: Foundational Knowledge for Testing Professionals Steve Saladin, Ph.D. University of Idaho."— Presentation transcript:

1 Psychometrics 101: Foundational Knowledge for Testing Professionals Steve Saladin, Ph.D. University of Idaho

2 What we are going to cover Understanding test scoresDamn the statistics & full speed ahead! Accuracy of the testError, error everywhere so whats my score? ValidityTo use or not to use, that is the question!

3 Criterion-referenced vs norm- referenced Is performance rated on some pre-established cut points or is it based on comparisons with others Class room grading is generally criterion based 90% right=A, 80%=B, 70%=C, etc. Typically reported as a percentage correct or P/F Grading on the curve means grade based on comparison with rest of class (norm-referenced) 80% might be a B, an A, a C or something else.

4 Standardized tests are typically norm-referenced SAT, ACT, GRE, IQ test Typically reported as percentile or standard score Certification exams are often criterion-referenced Proctor certification, licensing exams Typically reported as percentage correct or P/F Sometimes you get a mix GED uses norms to establish cut-scores Important to note difference between percentile and percentage correct Criterion-referenced vs norm- referenced

5 Damn the Statistics & full speed ahead! Testing is all about quantifying something about people (skills, knowledge, behavior, etc.) Stats are just a way to describe the numbers Make it more understandable Reveal relationships To understand norm-referenced test scores, you need to know two general things What is the typical score? To what degree did others score differently?

6 Whats typical? Mean Median Mode How different are the scores? Range = highest – lowest = 40 Variance = average of squared differences from mean = 163.6 Standard Deviation = square root of Variance = 12.8 10 10 20 20 30 30 40 40 40 40 50 = arithmetic average = 30 = # in the middle = 30 = most frequently occurring # = 40

7 Standard Normal Distribution Normal Curve Assumes trait is normally distributed in population Mean Standard deviation

8 Examples SAT/GRE scores are based on a scale of 200- 800 Mean = 500 SD = 100 The Wechsler IQ test Mean = 100 SD = 15 ACT scores range from 1 – 36 Mean = 18 SD = 6

9 The Normal Curve %tile <1% 2.5% 16% 50% 84% 97.5% 99.5% GRE 200 300 400 500 600 700 800 SAT 200 300 400 500 600 700 800 IQ 55 70 85 100 115 130 145 ACT 1 6 12 18 24 30 36

10 How are these things related? GRE scores and Grad School grades CLEP scores and final exam scores Compass/Accuplacer scores and success in entry classes Motivation and cheating Correlation tells us if things vary or change in a related way Higher GRE scores means higher grades Lower motivation suggests higher levels of cheating

11 Some Facts About Correlation Ranges from +1.0 to -1.0 Sign tells you direction of correlation + as A gets bigger so does B - as A gets bigger, B gets smaller

12 How To Lie With Statistics! Test Taking linked to Longevity! A recent study found that people who had taken more tests during early adulthood tended to live longer. The number of tests taken between the ages of 16 and 30 correlated strongly with the age of death. The more tests you take, the longer you will live!

13 Some Facts About Correlation It is not causation, but can be used to predict Small samples may miss relationship Heterogeneous samples may miss relationship 0.87 0.78 0.42

14 Error, Error Everywhere No test is perfect, no measurement is perfect ________ Get more precise, but never get exact Score = Truth + Error

15 Error, Error Everywhere Error can be lots of things including The environment The test-taker Procedural variations The test itself Since error makes scores inconsistent or unreliable, a measure of reliability of scores is important

16 Reliability Test-Retest Test group on two different occasions and correlate the results Are results stable over time Internal Consistency Correlate score on each item to total Are they all measuring the same thing Alternate Forms Develop two versions of same test and correlate scores on each Are your versions comparable All correlations so subject to same problems

17 So whats good? GRE has reported reliability of 0.89 (Quantitative), 0.92 (Verbal) GRE Guide to Use of Scores, 2007-2008 ACT Technical Manual reports Composite score reliability of.97 SAT reports reliabilities of.89-.93 Test Caharacteristics of the SAT on COMPASS alternate forms reliability reported to be.73-.90 d.pdf

18 Reliability & Error

19 Theoretical distribution of scores |------68%------| |----------------96%---------------| | 2% 14% 34% 34% 14% 2% -3 -2 -1 0 +1 +2 +3 2% 16% 50% 84% 96% 1 SEM below to 1 SEM above = 68% confidence 2 SEM below to 2 SEM above = 95% confidence

20 SEM for some tests GRE Verbal.34, Quantitative.51, so 68% confidence interval for score of 500 is 470-530 for Verbal, 450-550 for Quantitative Only reported in increments of 10 GRE Guide to Use of Scores, 2007-2008 ACT Composite SEM.91, so 68% confidence interval for score of 20 is 19-21 ACT Technical Manual WAIS-IV FSIQ SEM is 2.16, so 68 % confidence interval for score of 100 is 98-102

21 Does Reliability = Validity? Getting a consistent result means reliability Having that result be meaningful is validity Validity is based on inferences you make from results Test has to be reliable to be valid Test does not have to be valid to be reliable NO !

22 Validity Any evidence that a test measures what it says it is measuring Any evidence that inferences made from the test are useful and meaningful 3 types of evidence Content Criterion-Related Construct

23 Content Validity Think of a test as a sample of possible problems/items 4 th grade spelling test should be a representative sample of 4 th grade spelling words GRE Quantitative should be a representative sample of the math problems a grad school applicant might be expected to solve Should be part of design Identifying # of algebra, trig, calculus, etc. should be on test (table of specifications) Frequently evaluated by item analysis or expert opinions

24 Criterion-Related Validity How does test score correlate with some external measure (criterion) Placement test score and performance in class Admission test score and GPA for first semester Sometimes called Predictive or Concurrent Validity Correlation that is effected by error in the test and error in the criterion Only top students take GRE Graduate School grade restriction

25 To use or not to use…. Depends on the question…. What is impact of decision? What is cost of using? Of not using? Decision Theory can be a guide to determining incremental validity Net gain in using scores

26 Decision Theory False negativeTrue positive True negativeFalse Positive GPAGPA GRE score 200400600800 ABCABC Maximize success

27 Decision Theory False negativeTrue positive True negativeFalse Positive GPAGPA GRE score 200400600800 ABCABC Maximize opportunity

28 Predictive Utility Effectiveness = True Positive + True Negative True Pos+False Pos+True Neg+False Neg Have to weigh effectiveness against cost

29 Construct Validity Most important for psychological test where what you are measuring is abstract or theoretical Intelligence Personality characteristics Attitudes and beliefs Usually involves multiple pieces of evidence

30 Construct Validity Convergentcorrelates with measures of same thing Divergentdoes not correlate with measures of something else Scores show expected changes after treatment, education, maturation, etc. Factor analysis supports expected factor structure

31 Things to remember The normal curve Correlation Reliability Standard Error of the Measurement Validity Decision Theory

32 Not all scales are created equal Nominal ScaleSex, Race Ordinal ScalePercentile Rank, Letter Grades Interval ScalesIQ, temperature, SAT Ratio Scalesspeed, weight

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