..-
help for ^cid^ (statalist Mar. 20, 1998)
..-
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^aired ^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 and
varname2. Data are assumed to be paired, but ^unpaired^ changes this assumption.
In the third form, ^cid^ gives a CI for the mean difference across the two groups
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@.