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Question 3: Intra-operator ICCs varied from 0.83 to 0.95 for the experienced operators and from 0.47 to 0.83 for the inexperienced. Why might we expect more experienced operators to have higher ICCs?

The intra-observer ICC includes the variance between subjects, i.e. the variance of subject’s true nuchal translucency, and the variance of repeated measurements on the same subject by the same observer. The ICC is equal to the variance between subjects divided by the variance between subjects plus the variance of repeated measurements.

The variance between subjects should be the same for all observers, but the variance of repeated measurements may be large for inexperienced observers. Hence the second part of the ICC ratio will be larger for inexperienced observers while the first part is constant. The ICC will therefore be smaller for inexperienced observers.

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Last updated: 21 April, 2008.