Stats
How many charts should I pull? (March 30, 2006).
Category: Sample size justification
I got a question from someone doing a quality review. She needs to pull a certain number
of medical records out of 892 and see whether the doctors followed the clinical guidelines
properly. The question is how to determine the proper number of charts to pull.
Since the outcome variable is binary (followed guidelines, deviated from guidelines), you
would estimate a proportion and place 95% confidence limits around that proportion. The
desired width of the confidence interval will tell you how many records you would need. So,
for example, if you suspected that 80% of the doctors followed the guidelines, and you would
be happy if your confidence interval around this estimate had a width of plus/minus 10% (that
is, the interval would go from 70% to 90%), then you would need to sample 62 patients.
There's a simple formula that you could use to get this value
where H is the half width of the confidence interval and Z is the appropriate percentile
from a normal distribution (1.96 for a 95% interval, for example).
You could also just find a program or a spreadsheet that does confidence interval
calculations and use trial and error to find the right sample size. I have a simple
spreadsheet that will work
or you could find something on the web. A nice general resource for pages that perform
statistical calculations is
One wrinkle in the ointment is that only some of the charts that you pull will actually be
relevant to your inquiry and there is no way of telling which charts will work without
reviewing the entire chart. If you believe that only one out of every four charts meets your
needs, then you should pull 248 charts to make sure that you will have around 62 that qualify
when you are done.
07/08/2008.
