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We studied the precision and accuracy of SF measurements. Five trained obstetricians made three measurements of SF on each of 33 patients in the third trimester. The intraobserver variation varied from 0 cm to 8 cm, and comprised on average 1.5 to 2 cm (95th centiles 2.5 to 4 cm). The interobserver variation varied from 1 to 13 cm, and comprised on average 4 cm (95th centile 8 cm). The accuracy of SF measurements was estimated by comparison with ultrasound-guided measurements, which showed a very high precision. Two of the investigators differed significantly, by -1.56 cm and -1.97 cm, from the other 3.
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Bagger et al. (1985) studied this question.
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