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Cohen's kappa
Cohen's kappa is a score that shows how far 2 coders agree. It runs from -1 to 1. The score takes away the agreement that luck alone would bring. A kappa of 0 means the 2 coders agree no more than chance.
Example: 2 coders mark teacher gaze in the same lesson. They work apart, on the same video and the same track. Their kappa is 0.72. That is a good sign that the coding guide is clear.
Why it matters in GRAPE MARS
The Inter-rater Agreement module calculates kappa for you. It needs exactly 1 video in scope and 2 coders. Many researchers read the score with the bands of Landis and Koch. Below 0 is worse than chance. 0.41 to 0.60 is moderate. 0.61 to 0.80 is substantial. Above 0.80 is almost perfect. These bands are a common habit in the field, not a rule.