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Lecture 7: Item Analysis

Lecture 7: Item Analysis. PSY 605. Item Analysis - Overview. What is it? Item-level analysis of psychometric properties Assessment of how each item contributes to the reliability and validity of the scale Method of determining which items to cut/edit When do we do it?

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Lecture 7: Item Analysis

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  1. Lecture 7: Item Analysis PSY 605

  2. Item Analysis - Overview • What is it? • Item-level analysis of psychometric properties • Assessment of how each item contributes to the reliability and validity of the scale • Method of determining which items to cut/edit • When do we do it? • Before and/or during validation study • Ideal: • Study 1 – Pilot Test, Item Analysis, Revisions • Study 2 – Validation Study, (more) Item Analysis, (more) Revisions • Study 3 – (another) Validation Study • Typical: all 3 at once

  3. Qualitative & Quantitative Analyses • Qualitative (old news) • Content adequacy & dimension fit judgments • Done through good scale development process • Quantitative (today’s focus) • Item Difficulty (i.e., Item Endorsement) • Item Discrimination • Item Homogeneity (Impact on Scale Reliability) • Factor Loadings

  4. Overview of Quantitative Item Analyses

  5. Item Difficulty • Item difficulty index (aka item endorsement index) = proportion of test takers responding correctly (or endorsing the item) • Higher value = easier item/ less extreme item • Knowledge tests: based on classical test theory assumptions, aim for difficulty levels around .5 • Attitude/perception tests • Ideal value dependent on purpose • Use p(agree or strongly agree) and/or mean score

  6. Item Discrimination • Extent to which an item can discriminate (i.e., separate) test-takers with high overall test scores from those with low overall test scores • Groups can be top/bottom 10%, top/bottom thirds…

  7. Item Homogeneity • Inter-item correlations • Correlations between all possible item pairs (viewed as matrix) • looking for moderately strong, positive values • negative values may indicate forgotten reverse-coding or bad item • very strong values may indicate unnecessary redundance • Scale Alpha when item removed • What Cronbach’s Alpha would be if the scale did not contain that item (and, thus, was 1 item shorter) • A good item: Alpha drops significantly when item removed • A bad or unnecessary item: Alpha increases or doesn’t change much when item removed

  8. Factor Loadings • A step back – a conceptual look at factor analysis Latent Construct B Latent Construct A Item 1 Item 2 Item 7 Item 3 Item 8 Item 4 Item 5 Item 6

  9. Factor Loadings Latent Construct A Latent Construct B Item 1 Item 3 Item 5 Item 7 Item 6 Item 8 Item 2 Item 4

  10. Factor Loadings – what you hope to see Latent Construct A Latent Construct B .5 .6 .8 .2 .2 .1 Item 1 Item 3 Item 5 Item 7 .9 .3 Item 6 Item 8 Item 2 Item 4

  11. Factor Loadings – what you hope to see Latent Construct A Latent Construct B .9 .7 .1 .2 .5 Item 1 Item 3 Item 5 Item 7 .8 .2 .1 Item 6 Item 8 Item 2 Item 4

  12. Factor Loadings – if you see… Latent Construct A Latent Construct B .9 .7 .1 .2 .5 Item 1 Item 3 Item 5 Item 7 -.7 .2 .1 Item 6 Item 8 Item 2 Item 4

  13. Factor Loadings – if you see… Latent Construct A Latent Construct B .9 .7 .6 .2 .5 Item 1 Item 3 Item 5 Item 7 .2 .2 .1 Item 6 Item 8 Item 2 Item 4

  14. A Full Example: Item Analysis and the Organizational Injustice Survey

  15. Item Endorsement: Item Means & Variances

  16. Item Impact on Scale Reliability:Inter-Item Correlations

  17. Item Impact on Scale Reliability: Alpha when item removed Injustice in Opportunities & Outcomes: α = .913 Injustice in Guidelines & Expectations: α = .904 Injustice in Interpersonal Treatment: α = .913

  18. Factor Analysis: Principal Axis Factor Analysis with Direct Oblimin Rotation

  19. Factor Analysis: Principal Axis Factor Analysis with Direct Oblimin Rotation

  20. Factor Analysis: Principal Axis Factor Analysis with Direct Oblimin Rotation Items 1-7: “Injustice in Opportunities & Outcomes” Items 8-15:”Injustice in Guidelines & Expectations” Items 16-24: “Injustice in Interpersonal Treatment”

  21. Determining Item Edits & Deletions

  22. Final Scale • 6 items cut that violated the majority of the ‘good item’ rules • Final measure: 18 items; 6 items for each of the 3 subscales • Subscale Internal Consistency Reliabilities: • Outcomes & Opportunities: .90 • Guidelines & Expectations: .88 • Interpersonal Treatment: .90 • NOW we collect validation evidence of the final scale, using all the strategies discussed in lectures 5 & 6.

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