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At my institution, we have 40 patients with a certain medical condition. For a particular variable, lets say "days in hospital" we have a mean (3.2 days). We'd love to compare that to previously reported data.

For instance, in one meta-analysis, the pooled mean from multiple studies for that variable was 6.4 days.

So my question is: is it safe to use the 6.4 as a "population mean", and then if so, can I compare my 3.2 to it? If so, silly question but that'd be a one sample t-test correct?

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When you compare the means, one important parameter you need to have is variance. Since you are using a mean value from multiple studies, I don't think it is a good idea. Of course, you can use the pooled mean, but you need to use pooled variance: http://sites.gsu.edu/psyc3530/independent-samples-t-test/independent-t-tests/

Another way to avoid the issue is to apply an ANOVA analysis, which allows you to compare multiple means from the different studies. "A one-way ANOVA compares three or more than three categorical groups to establish whether there is a difference between them. Within each group there should be three or more observations (here, this means walruses), and the means of the samples are compared."

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