The McNemar Index can be used to identify bias in the disagreements between 2 observers making binary observations. This is a simple index based on the premise that random disagreements should have an equal occurrence.

Observer 2

Observer 1





agree 1

disagree 1


disagree 2

agree 2


McNemar index =

= ((disagree 1) - (disagree 2)) / ((disagree 1) + (disagree 2))



• minimum index: -1

• maximum index: 1

• The closer the index is to 0 the less bias there is between the observers.

• The higher the absolute value for the index the greater the differences between the observers and the greater the bias.

• Bias in observer 1 results in a positive index while bias in observer 2 results in a negative index.



• The number of observations that agree are not considered.

• The index can reach a maximal value with a disagreement pair of 1 and 0.

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