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NBME 21 Answers

nbme21/Block 4/Question#17

A 48-year old man is referred for evaluation of ...

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Confidence interval increases with decreased sample size.

usmleuser007  would require a a large sample size to see if there is a true difference +  
claptain  This question is bogus. CI does not always increase with decreased sample size or vice versa. Four readings with small variation would give a narrower CI than 10 readings with greater variation. The only thing you can be certain about by adding more samples is that the CI will most likely change, but which direction is uncertain. +1  
bartolomoose  Recall the formula for 95%ci Mean +/- 1.96* (SD/sqrt(samplesize)) +1  




Remember that the CI= X +/- Z(SE) where SE=(SD/n^.5) and SE decreases as n increases, and vice versa. In our question, n, the sample size, is decreasing because we will do fewer measurements of the BP. This means SE will increase, so the CI will increase.

Basically, with fewer measurements, the CI gets wider because you need a larger sample size for more valuable results.

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