Exercise: observer agreement about eye colour

This website is for students following the M.Sc. in Evidence Based Practice at the University of York.

183 students were observed twice by different student observers. These measured height (mm), arm circumference (mm), head circumference, and pulse (beats/min) and recorded sex and eye colour (black, brown, blue, grey, hazel, green, other). They entered these into a computer file. Eye colour and sex were entered as numerical codes.

The following table shows eye colour recorded by the two observers:

Eye colour recorded   by first observer	Eye colour recorded by second observer							Total
	black	brown	blue	grey	hazel	green	other
black	6	4	0	0	0	0	0	10
brown	6	69	0	0	4	0	1	80
blue	0	0	39	1	0	2	2	44
grey	0	1	1	4	0	4	0	10
hazel	0	1	0	0	9	4	0	14
green	0	0	1	1	1	15	2	20
other	0	0	0	0	0	2	3	5
Total	12	75	41	6	14	27	8	183

The Stata output for the kappa statistic for this table is:

. kap eye1 eye2

             Expected
Agreement   Agreement     Kappa   Std. Err.         Z      Prob>Z
-----------------------------------------------------------------
  79.23%      26.16%     0.7188     0.0385      18.69      0.0000

Question 1:

How would you describe the level of agreement in this table?

Check suggested answer 1.

Question 2:

The expected agreement is much lower than for sex, where it was 54.52%. Why is this?

Check suggested answer 2.

Question 3:

How could we improve the kappa statistic?

Check suggested answer 3.

Question 4:

What pairs of categories might be regarded as minor disagreements?

Check suggested answer 4.

Question 5:

What might be plausible weights for the pairs of eye colour categories?

Check suggested answer 5.

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Last updated: 21 July, 2008.

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