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st: Treatment of odds ratios for tables with a 0 cell frequency
I just had occasion to notice that commands that produce confidence
limits for odds ratios in Stata (e.g., cc, mhodds) decline to
calculate exact confidence limits when there is a zero cell in the
table. For example,
-cci 10 20 30 0- yields "Note: exact confidence levels not possible
with zero count cells." To my knowledge, this is not the practice in
other programs that calculate exact CIs for odds ratios, such as
StatXact: they will set the lower limit to 0 and calculate an exact
upper limit, or set the upper limit as undefined and calculate an
exact lower limit. Since there is no particular algorithmic
difficulty in doing this calculation (in contrast to the error
message), does anyone have an idea of why the choice not to calculate
these limits has been implemented in Stata? I personally think it is
useful to know, for example, what the lower confidence limit is on
the OR even when the observed value is infinite due to a zero cell.
=-=-=-=-=-=-=-=-=-=-=-=-=
Mike Lacy
Fort Collins CO USA
(970) 491-6721 office
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