This page contains only historical information and is not about the current
release of Stata.
Please see our Stata 12 page
for information on the current version of Stata.
Exact statistics
Stata 10 offers several commands for exact analysis, providing more reliable
statistical inference with small-sample datasets.
- New command
exlogistic
fits exact logistic regression models.
The dependent variable can be Bernoulli (0 or 1) or binomial
(the number of successes in n
trials). Exact joint hypothesis tests can be performed, and
predictions with exact confidence intervals can be obtained.
- New command expoisson
fits exact Poisson regression models of count outcomes. Exact joint hypothesis tests
can be performed.
- Other commands for exact statistics include
| Command |
Description |
|---|
| bitest |
Binomial probability test |
|---|
| cc |
Case–control study odds ratios |
|---|
| ci |
Centiles with confidence intervals |
|---|
| cs |
Cohort study risk differences and ratios |
|---|
| ir |
incidence-rate ratios and differences |
|---|
| istdize |
Indirect standardization |
|---|
| ksmirnov |
Two-sample Kolmogorov–Smirnov test |
|---|
| loneway |
Large one-way analysis of variance |
|---|
| mcc |
Matched case–control study statistics |
|---|
| median |
Equality-of-medians test |
|---|
| symmetry |
Symmetry and marginal homogeneity tests |
|---|
| tabulate2, tab2 |
Two-way tables with Fisher’s exact test |
|---|
| tetrachoric |
Tetrachoric correlations |
|---|
Exact logistic example
Stata’s new exact logistic regression command provides better coverage
in small samples than does standard logistic regression.
It also provides parameter estimates and confidence intervals where standard
asymptotic methods cannot.
Such cases include small-data problems with binary regressors for which the
outcome is 1 whenever the regressor is 1.
In the example below, every treated patient exhibits a positive response.
Standard logistic regression cannot estimate the treatment effect. Stata
10’s new exlogistic command can:
. exlogistic response treatment x2 x3
Parameter estimates, standard errors, and CIs are calculated on the basis of
permutation without recourse to asymptotic assumptions and results.
Not only is the treatment effect estimated, but tests of significance and
reported confidence intervals are based on exact methods.
Stata 10 also includes exact Poisson regression for count data.
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