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Study Design and Analysis in Neuroradiology: A Practical Approach

L. Santiago Medinaa

a From the Sections of Health Services and Policy and Pediatric Neuroradiology, Department of Radiology, Children's Hospital Medical Center, 3333 Burnet Ave, Cincinnati, OH 45229.



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  		FIG 1. A and B, Test with a low (A) and high (B) threshold. The sensitivity and specificity of a test changes according to the threshold selected; hence, these diagnostic performance parameters are threshold-dependent. Sensitivity with a low threshold (TPa/diseased patients) is greater than sensitivity with a higher threshold (TPb/diseased patients). Specificity with a low threshold (TNa/nondiseased patients) is less than specificity with a high threshold (TNb/nondiseased patients). TP indicates true positive; TN, true negative; FP, false positive; FN, false negative.



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  		FIG 2. A–C, The perfect test (A) has an area under the curve (AUC) of 1. The useless test (B) has an AUC of 0.5. The typical test (C) has an AUC between 0.5 and 1. The greater the AUC (ie, excellent > good > fair), the better the diagnostic performance



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  		FIG 3. Validity and reliability of a test.

A, Good validity and reliability means the center of the target is always hit by different gunmen.

B, Good reliability but poor validity means the same area of the target is hit by different gunmen but off center.

C, Poor reliability of the test means the bullets are spread out, all over the target, with a few hitting the center. In the hands of a few gunmen a test can be valid, since the center of the target is hit.



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  		FIG 4. A and B, Although the mean is the same for study populations of both small (A) and large (B) patient samples, the CI narrows as the number of subjects increases.



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  		FIG 5. Flow diagram summarizes the key steps required to do sound clinical research





Copyright © 2008 by the American Society of Neuroradiology. Print ISSN: 0195-6108 Online ISSN: 1936-959X