Question 4: The inter-operator repeatability, showed a variation in ICC from 0.74 to 0.95 in pairs of experienced sonographers to 0.51 in one pair inexperienced with the measurement. Why is the inter-operator ICC less than the intra-operator ICC, irrespective of the experience of the operators?

The inter-operator ICC is equal to the variance between subjects divided by the variance between subjects plus the variance of repeated measurements by different observers. The intra-operator ICC is equal to the variance between subjects divided by the variance between subjects plus the variance of repeated measurements by the same observer. Because there is extra error when different observers make measurements, the variance of repeated measurements by different observers is greater than the variance of repeated measurements by the same observer. Hence the second part of the ICC ratio will be larger for different observers than for the same observer. The ICC will therefore be smaller for inter-observer differences.

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Last updated: 24 July, 2008.