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  • Difference in CI calculation - why?

    Hi,

    I was using the mean command to calculate confidence intervals and I found out that there is a difference in CI calculation:

    #
    cls
    clear
    version 14.1
    input var group
    15 1
    19 1
    17 1
    13 1
    12 2
    18 2
    11 2
    19 2
    end

    mean var if group == 1
    mean var, over(group)
    #

    Using the command with "if" I get a CI (-/+ 95%) of 11.89148 to 20.10852
    Using the "over" command I get: 12.94728 to 19.05272

    All calculations have the same standard error. As I have found out in Stata help is that "over" specifies that estimates are computed for multiple subpopulations, with special formula for subpopulation estimation. My question is: why would I need a special subpopulation calculation vor CI over groups?

    -Nick
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