```..-
help for ^cid^                                          (statalist Mar. 20, 199
> 8)
..-

Confidence intervals for differences
- ------------------------------------

^cid^  varname [^if^ exp] [^in^ range] [^, m^edian ^l^evel^(^#^)^ ]

^cid^  varname1 varname2 [^if^ exp] [^in^ range] [^, m^edian ^unp^aired ^une^
> qual
^l^evel^(^#^)^ ]

^cid^  varname [^if^ exp] [^in^ range]^, by(^groupvar^)^ [ ^m^edian ^unp^aire
> d ^une^qual
^l^evel^(^#^)^ ]

^cid^ calculates confidence intervals (CIs) for means or differences.

In the first form, ^cid^ calculates a CI for the mean of varname.

In the second form, ^cid^ produces a CI for mean difference between varname1 an
> d
varname2.  Data are assumed to be paired, but ^unpaired^ changes this assumptio
> n.

In the third form, ^cid^ gives a CI for the mean difference across the two grou
> ps
defined by groupvar.  Data are assumed paired, but ^unpaired^ reverses this.

Options
- -------

^median^ calculates CI's as above, except for medians rather than for means.
CI's are determined by rank-based methods as described in Gardner and
Altman's (1989) book "Statistics with Confidence", pp. 74-79.  A normal
approximation with a continuity correction of 0.5 is used, rather than
tables of exact values.  The accuracy should be adequate when both groups
have at least 5 observations.

Note that the CI for a single median is obtained using a method different
from that used by the @centile@ command, so will not be identical.

WARNING: calculation of CIs for unpaired medians requires storage space for
n1 * n2 observations, where n1 and n2 are the numbers in each group.  If
you have insufficient memory, you will get an error message and a ^r(2002);
> ^
return code.

^unpaired^ indicates that the data are to be treated as unpaired.

^unequal^ indicates that the unpaired data are not to be assumed to have equal
variances.

^level(^#^)^ defines the percent coverage for the confidence interval.

Examples
- --------

. ^cid bp2 bp1^
. ^cid bp2 bp1, median^
. ^cid bp2 bp1, median unpaired^
. ^cid bp, by(treatmnt)^
. ^cid bp, by(treatmnt) median^
. ^cid bp, by(treatmnt) unpaired^
. ^cid bp, by(treatmnt) unpaired unequal^

Stored
- ------

^\$S_1^  number of observations
^\$S_2^  degrees of freedom (not used if ^median^ is specified)
^\$S_3^  mean or median difference, group 1 - group 2
^\$S_4^  SEM, or K if ^median^ specified (see Gardner and Altman)
^\$S_5^  lower confidence limit for difference
^\$S_6^  upper confidence limit for difference

Also see
- --------

Manual:  [R] @ci@, @centile@, @ttest@, @ranksum@, @signrank@.

```