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Comprehensive Guide to Scales, Indices, and Measurement Overview

Explore the two-step process of using multiple indicators to create variables, including Factor Analysis and evaluating reliability with Cronbach’s Alpha. Learn how to conduct Concept Measurement and Example of Factor Analysis in research studies.

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Comprehensive Guide to Scales, Indices, and Measurement Overview

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  1. Scales & Indices

  2. MeasurementOverview • Using multiple indicators to create variables • Two-step process: • 1. Which items go together to measure which variables • Factor Analysis • 2. Evaluating the reliability of multi-item scales • Cronbach’s Alpha

  3. Factor Analysis • Starts with a group of similar indicators (survey items) • Sorts items based on patterns of inter-item similarities • I.e., which items are correlated (which ones group together) • Items that group together share some underlying common underlying factor • Procedure is based on inter-item correlations • Correlation: • Measure of similarity between two variables • Varies between 1 and -1

  4. Stages in Factor Analysis • Extraction • How the computer searches for patterns • Rotation • Mathematical manipulation of patterns • Whether the computer produces correlated or uncorrelated factors

  5. Concept measurement example: • Research on effects of TV news coverage of social protest • Subjects shown one of three TV news stories about an anarchist protest: • 1. Extremely critical • 2. Highly critical • 3. Moderately critical • Respond to questionnaire • Examined differences between exposure groups

  6. Example of Factor Analysis • Started with 28 items measuring attitudes • Factor analysis reduces to underlying factors…

  7. Remove

  8. Remove

  9. Remove

  10. Five Factors • 1. Protest rights • 2. Police hostility • 3. Protest utility • 4. Blame the protesters • 5. Anti-violence

  11. 1. Support for Protest Rights • A. Protesters have a right to protest • B. Protesters should not be allowed to protest in public places (reverse coded) • C. Protesters have a right to be heard

  12. 2. Hostility the Police • A. Police were out of line • B. Police used excessive force • C. Police were violent

  13. 3. Utility of Protest • A. Protesters offer new insights • B. It’s important to listen to protesters • C. Protesters brought issues to my attention

  14. 4. Blame the Protesters • A. Protesters initiated the conflict • B. The protesters were disrespectful • C. Protest was ineffective on politicians

  15. 5. Opposition to Protest Violence • A. I feel sorry for the police because of the way they were treated by the protesters • B. The protesters were violent

  16. Combining items into a scale • Summative scale • Factor scores

  17. Summative scales • Adding items or taking the mean • E.g.,: • Compute scale = sum.1(var1,var2,var3) • Compute scale = mean.1(var1,var2,var3) • Weights each item equally

  18. Factor scores • Uses factor loadings from the factor matrix to weight the items • Heavier weighting to items that are more central to the factor • Use save command when running factor analysis (under “scores”: “save as variables” • New variables with values for each case saved in data file for each factor

  19. Cronbach’s Alpha • Assessing reliability of a multi-item scale • Based on the average inter-item correlation • Weighted by the number of items in the scale • Measures internal consistency (unidimensionality) • Are all the items measuring the same thing? • If so, they should all be highly inter-correlated

  20. Cronbach’s Alpha Formula: A = N * r [1+ (N –1)r] N = number of items in the scale r = average inter-item correlation

  21. Acceptable alpha for a scale • Ideally, alpha > .80 • Some journals accept > .70 • Low alpha means either: • 1. Scale is not reliable (items have lots of error) • 2. Items could measure two different things • Alpha if item deleted can help identify a bad item • More than one bad item could be an indicator that there are items that measure a different concept

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