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UNL PSYC 971 - Practical Psychometrics

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Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Practical Psychometrics• Preliminary Decisions• Components of an item• # Items & Response• Approach to the Validation ProcessThe items you need and the validation processes you will choose all depend upon “what kind of scale you are writing” – you have to decide …• measuring, predicting, or measuring to predict ?• construct / content ? • quantifying or classifying or multiple classifications ?• target population ?• single scale or multiple subscales ?• want face validity ?• relative/rank or value reliability ?• alternate forms ?Components of a single item…Item = target construct + systematic error + random errorSystematic error sources• other constructs• social desirability / impression management• asymmetric response scales (e.g., average, good, great, awesome)Random error sources • non-singular (double-barreled) items• response patterns (e.g., answer all 5s)• inattention / disinterestItem-writingKind of item ?Judgement vs. Sentiment – what they know or what they think?Absolute vs. Comparative – what want them thinking about?Preference vs. Similarity – want ranking/selection or values?Things to consider …• don’t mix item types too much – confusing to respondents• consider what you are trying to measure• consider what you will do with the response values • are there “correct answers” or “indicative responses”• e.g., ratings are easier to work with than ranks•consider the cognitive abilities of the target populationItem Writing, cont. – focusing on sentiments…How many response options ? • binary – useful if respondents have “limited cognitive capacity”• 5 – the Likert standard (perhaps a historical accident?)• 7 +/- 2 – based on working memory capacityImportant Issue #1 – middle item ???• some don’t like allowing respondents to “hug the middle”• research tells us that, if given an even #responses, they will “hug the middle 2” and increase error varianceImportant Issue #2 – verbal anchoring ???• some like to have a verbal anchor for all items & other like to anchor the ends (semantic differential) & some also like to anchor the middle• some research has shown that labels are “less interval” than #s – i.e., the anchors “hurt” getting interval-like dataImportant Issue #3 – Item SensitivityItem sensitivity relates to how much precision we get from a single item• Consider a binary item with the responses “good” & “bad” -- big difference between a “1” and a “2”• Consider a 3-response item with “dislike” “neutral” “like” – huge “steps” among 1,2 & 3 – can lead to “position hugging”• Consider a 5 – response item “strongly disagree” “disagree” “neutral” “agree” “strongly agree” – smaller “steps” among1,2,3,4 & 5 – should get less “hugging” and more sensitivity• Greater numbers of response options can increase itemsensitivity – beware overdoing it (see next page)Important Issue #4 – Scale sensitivityScale sensitivity is the “functional range” of the scale which is tied to the variability of data values• Consider a 5-item true-false test – available scores are 0% 20% 40% 60% 80% & 100% -- not much sensitivity & lots of tiesHow to increase scale sensitivity?• Increase #responses/item sensitivity – can only push this so far• Increase # items – known to help with internal consistency• both – seems to be the best approachConsider:• 1 item with 100 response options (98 +/- 2???) (VAS?)• 100 binary items (not getting much from each of many items)• 50 items with 3 options (50-150)• 20 items with 6 options (20-120)• 12 items with 9 options (9-108)Working the #item - #responses trade-offImportant Issue #5 – Item & Scale difficulty / response probability“What” you are trying to measure from whom will impact “how hard” the items should be…• obvious for judgment items -- less obvious for sentiment • consider measuring “depression” from college students vs.psychiatric inpatients – measuring very different “levels” ofdepression“Where” you are measuring, “Why” you are measuring & from whom will impact “how hard” the items should be …• equidiscriminating math test for 3rd graders vs. college math majors• identifying “remedial students” math test for 3rd graders math majorsValidation ProcessOver the years several very different suggestions have been made about how to validate a scale – both in terms of the kinds of evidence that should be offered and the order in which they should be sought. Couple of things to notice…Many of the different suggests aren’t “competing” – they were suggested by folks working in different content areas with different measurement goals – know how scales are constructed and validated in your research area !Urgency must be carefully balanced with process – if you are trying to gather all the forms of evidence you’ve decided you need in a single or couple of studies you can be badly delayed if one or more don’t pan out…Desirable Properties of Psychological MeasuresInterpretability of Individual’s and Group’s ScoresPopulation Norms (Typical Scores)Validity (Consistent Accuracy)Reliability (Consistency)Standardization (Administration & Scoring)Process & Evidence ApproachesLet’s start with 2 that are very different…“Cattell/Likert Approach”• focus on criterion-related validity• requires a “gold standard” criterion – not great for “1st measures”• emphasizes the predictive nature of scales & validation• a valid measure is a combination of valid items• a scale is constructed of items that are each related to criterion• criterion-related validity coefficient is the major evidence• construct validation sometimes follows • tend to have “limited” internal consistency and “complex” factor structures – these are not selection or validation goalsProcess & Evidence Approaches“Nunnally Approach”& very different from that …• focus on content validity• does not require a “gold-standard” criterion • is the most common approach for “1st measures”• emphasizes the measurement nature of scales & validation• a valid measure is made up of items from the target content domain• internal

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