Inter-Rater Reliability

1. Inter-Rater Reliability Emily PhillipsGalloway William Johnston March 1, 2013

2. Accessing Workshop Materials Go to: isites.harvard.edu/research_technologies • Click on Workshopstab (on the left) and then theInter-Rater Reliability folder(near the bottom) • Save all of the files to the desktop (right click and ‘Save Link As’)

3. Agenda Introducing IRR What is Kappa? • ‘By-hand’ Example Limitations and Complications of Kappa Working Through a Complex Example • Data setup • Estimation • Interpretation Reporting Results

4. What is Inter-Rater Reliability? IRRcan be defined as the degree of agreement among raters. Numerous statistics can be calculated to provide a score of how much consensus exists between raters. • Why does it matter in educational research?: In IRR we trust • The quality of a coding scheme and the ability to replicate results is connected with the overall ‘believability’ of the results. To publish results, we must demonstrate that our coding scheme is reliable.

5. What is Inter-Rater Reliability? IRRcan be defined as the degree of agreement among raters. Numerous statistics can be calculated to provide a score of how much consensus exists between raters. • Why does it matter in language & literacy research? • Language data can be challenging to code and can fall prey to subjectivity given that interlocutors will not always say or write all that may be inferred by scorers.

6. What is Inter-Rater Reliability? IRRcan be defined as the degree of agreement among raters. Numerous statistics can be calculated to provide a score of how much consensus exists between raters. • Why does it matter in language & literacy research? • Language data can be challenging to code and can fall prey to subjectivity given that interlocutors will not always say or write all that may be inferred by scorers. Our Task: To design coding schemes that are not subjective.

7. IRR: A Beginning and an End How does formative IIR/inter-rater agreement tell us during the beginning / design phase of a study? If your coding scheme is being developed, calculating IRR can tell you if your codes are functioning in the same way across raters. If you are using an existing coding scheme, calculating IRR can tell you if your raters may need additional training.

8. IRR: A Beginning How does formative IIR/inter-rater agreement tell us during the beginning / design phase of a study? If your coding scheme is being developed, calculating IRR on 15%-20% of your data can tell you if your codes are functioning in the same way across raters. Disagreements are OK at this point! If there are numerous disagreements, this signals a need to revise your coding scheme (and recode the data), to locate clear examples to help raters to understand codes better, or (when all else fails) to abandon codes that do not function well.

9. IRR: A Beginning How does formative IIR/inter-rater agreement tell us during the beginning / design phase of a study? If are using an existing coding scheme, calculating IRR on 15%-20% of your data can tell you if your codes are functioning in the same way across raters. Disagreements are OK at this point! If there are numerous disagreements, this signals a need to retrain your raters (and recode the data) or to evaluate if the coding scheme you are working with may need to be revised for your data.

10. IRR (Agreement) by Hand! While kappa statistics give us insight into user disagreements on the aggregate, a ‘confusion matrix’ can help us to identify specific codes on which raters disagree. (Bakeman & Gottman, 1991)

11. Let’s try it Scenario: Marie and Janet are coding students’ definitions for the presence of nominalized words. For each definition, 0=no nominalizations and 1=nominalizations. They are at the beginning of scoring the data with a new coding scheme that Janet has developed. How does it seem to be working?

12. Are Janet & Marie still friends?

13. IRR: The Middle? How does IRR tell us during the middle of a study? In the middle of a study, especially if we are coding data over a long period, we may again conduct IRR analysis (a ‘reliability check’) to be sure we are still coding the data reliably. • involves selecting 20% of the data at random to assess for agreement.

14. IRR: An End Why does summative IRR matter during the analysis phase of a study? • At the end of a study, we calculate IRR to demonstrate to the academic community that our coding scheme functioned reliably. • Generally, if we have been diligent in developing our coding scheme and training our raters there are few surprises.

15. What is Kappa? Cohen’s Kappa (Cohen, 1960) is a numeric summation of agreement that accounts for agreements simply by chance. po = Proportion of agreement that is actually observed pc = Proportion of agreement by chance See pages 63-64 in the Bakeman & Gottman (1991) for an excellent example of how po, pc, and Cohen’s Kappaare calculated.

16. Limitations of Kappa What if there are more than two possible ratingsand the size of the discrepancy between raters matters? Use weighted Kappa What if there are more than two raters? Use Fleiss’ Kappa What if different participants have different numbers of raters? Use Krippendorff’s alpha

17. A Detailed Example The sample: 37 students The data structure: 8 key variables 2 different work explanation tasks • “bicycle” and “debate” 2 coders • coder1 and coder2 2 rating subscales • Super ordinate scale (0-5 points) • Syntax scale (0-6 points) Which statistic should we be using? Weighted Kappa! (p. 66 in B & G)

18. Estimating IRR in Stata Start with a simple cross tabulation: coder1_bic | ycle_super | coder2_bicyclel_superordinate ordinate | 0 2 3 4 5 | Total -----------+-------------------------------------------------------+---------- 0 | 13 0 0 1 0 | 14 2 | 0 5 0 0 0 | 5 3 | 0 0 3 0 0 | 3 4 | 0 0 0 5 0 | 5 5 | 0 0 0 0 10 | 10 -----------+-------------------------------------------------------+---------- Total | 13 5 3 6 10 | 37 • What do you notice? • Very strong agreement! Why might this be? • Neither reviewer ranked anybody a “1”. Why might this be? • This has implications for how we do this in Stata…

19. Estimating IRR in Stata Because of the nature of the ratings, we have to make some changes to the data in order for things to run. • Our weight matrix implies there are 6 possible ratings but only 5 are used by the raters  we must use the “absolute” option in Stata • BUT… in order for this to work we need to change the scale from 0,1,2…5 to 1,2,3…6. replace c1_bicycle_so = c1_bicycle_so+1 replace c2_bicycle_so = c2_bicycle_so+1 tab c1_bicycle_so c2_bicycle_so

20. Estimating IRR in Stata The command: kapc1_bicycle_so c2_bicycle_so, wgt(s_o_wgt) absolute The output: Expected Agreement Agreement Kappa Std. Err. Z Prob>Z ----------------------------------------------------------------- 97.84% 53.81% 0.9532 0.1284 7.43 0.0000

21. Apply Your Knowledge For each of the three remaining subscales repeat the steps: • Inspect a cross tabulation to get an idea of the data distributions • Create a weighting matrix • “w” or “w2” are built-in weight matrices that you can use (w is linear and w2 is quadratic) • Estimate the Kappa and interpret the results!

22. Estimating IRR with web resources If you do not have access to Stata, there are some great web resources you can use as well: http://www.stattools.net/CohenKappa_Exp.php http://www.agreestat.com/

23. Reporting Results “inter-rater reliability, calculated based on double coding of 20% of the tasks, was very high (Agreement = 98%; Cohen’s Kappa = .96).” (Kieffer & Lesaux, 2010) “To calculate inter-rater reliability for the coding scheme developed in the first phase, a research coordinator and a graduate research assistant randomly selected 20 writing samples for each of four tasks from different years, cohorts, and writing ability levels: narratives by pen, the sentence integrity task by pen, essays by keyboard, and the sentence integrity task by keyboard. One rater served as the anchor for computing percent agreement between coders. The inter-rater reliability was generally very good for each coded category. Except for two categories, initial percent agreement ranged from 84.6% to 100%. For the only two categories of low interrater reliability, subordinate and adverbial clauses, additional training and reliability checks improved inter-rater reliability for these to acceptable levels of over 0.80.” (Berninger, Nagy, & Scott, 2011)