rwolf calculates Romano and Wolf's (2005a,b) step-down adjusted p-values robust to multiple hypothesis testing. This program follows
the resampling algorithm described in Romano and Wolf (2016), and provides a p-value corresponding to the significance of a hypothesis
test where S tests have been implemented, providing strong control of the familywise error rate (the probability of committing any Type I
error among all of the S hypotheses tested). The rwolf algorithm constructs a null distribution for each of the S hypothesis tests based
on Studentized bootstrap replications of a subset of the tested variables. Full details of the procedure are described in
Romano and Wolf (2016), and further discussion of this program and its implementation, plus a full discussion of this ado, is provided in
Clarke, Romano and Wolf (2019).
There are two ways for this command to be used. First, either indepvar() and method() must be specified if the complete Romano-Wolf procedure should be implemented including the estimation of bootstrap
replications and generation of adjusted p-values. Alternatively, the user can provide rwolf with pre-computed bootstrap or permuted
replications of the estimated statistic and standard errors for each of their multiple hypothesis tests of interest. In this case, the
nobootstraps and pointestimates(numlist), stderrs(numlist) and stdests(varlist) should be indicated, and rwolf calculates the adjusted p-values from the replicates provided.
In the former case where rwolf takes care of estimating the bootstrap replicates of each test statistic and its standard error, rwolf simply
requires that the user indicates the multiple dependent variables to be tested, the independent variable of interest, and (optionally) a series
of control variables which should be included in each test. rwolf works with any estimation-based regression command allowed in Stata,
which should be indicated using the method() option. If not specified, regress is assumed. In the case that ivregress is specified, it is
assumed that the independent variable is the endogenous variable, and the instrumental variable(s) should be indicated in the iv() option. If
this is not the case (ie if the treatment variable is an exogenous variable in the IV model), this should be indicated with the indepexog
option. Optionally, regression weights, if or in can be specified. By default, 100 bootstrap replications are run for each of the S multiple
hypotheses. Where possible, a larger number of replications should be preferred given that p-values are computed by comparing estimates to a
bootstrapped null distribution constructed from these replications. The number of replications is set using the reps(#) option, and to
replicate results, the seed(#) should be set.
In the case of more complex situations where a user wishes to pre-compute their test statistics, standard errors, and a large number of bootstrap replicates of each these, the user can request for only the p-value correction algorithm to be implemented with the bootstrap option. This allows for cases where different estimation methodologies or different independent variables are used in each model within the family of hypothesis tests, or where more complicated resampling procedures are used, such as those based on permutation.
By default, the re-sampled null distributions are formed using a simple bootstrap procedure. However, more complex stratified and/or clustered
resampling procedures can be specified using the strata() and cluster() options. The cluster() option refers only to the resampling procedure,
and not to the standard errors estimated in each original regression model. If the standard variance estimator is not desired for
regression models, this should be indicated using the same vce() specification as in the original regression models, for example
vce(cluster clustvar). It is suggested that the cluster() and vce(cluster clustvar) should be used together.
The command returns the Romano Wolf p-value corresponding to each variable, standard (bootstrapped) uncorrected p-values, and for
reference, the original uncorrected (analytical) p-value from the initial tests when rwolf estimates baseline regression models. rwolf
is an e-class command, and the Romano Wolf p-value for each variable is returned as a scalar in e(rw_varname). A matrix is also returned as
e(RW) providing the full set of Romano-Wolf corrected p-values.
To install directly into Stata:
ssc install rwolf, replaceor using net install command:
net install rwolf, from("https://raw.githubusercontent.com/damiancclarke/rwolf/master") replace rwolf depvars [if] [in] [weight], [options]- indepvar(varlist) Indicates the independent (treatment) variable which is included in multiple hypothesis tests. This will typically be a single independent variable, however it is possible to indicate various independent (treatment) variables which are included in the same model, and the Romano-Wolf procedure will be implemented efficiently returning p-values for each dependent variable of interest, corresponding to each of the specified independent variables. This option must be specified, unless the nobootstraps option is indicated.
- method(regress | logit | probit | ivregress |...) Indicates to Stata how each of the multiple hypothesis tests are performed (ie the baseline models). Any estimation command permitted by Stata can be included. See regress for a full list of estimation commands in Stata. If not specified, regress is assumed. If an IV regression is desired, this must be specified with ivregress only, and the iv() option below must be specified.
- controls(varlist) Lists all other control variables which are to be included in the model to be tested multiple times. Any variable format accepted by varlist is permitted including time series and factor variables.
- nulls(numlist) Indicates the parameter values of interest used in each test. If specified, a single scalar value should be indicated for each of the multiple hypotheses tested, and these should be listed in the same order that variables are listed as depvars in the command syntax. In the case that multiple indepvars are specified, null parameters should be specified grouped first by indepvars and then by depvars. For example, if two independent variables are considered with four dependent variables, first the four null parameters associated with the first independent variable should be listed, followed by the four null parameters associated with the second independent variable. If this option is not used, it is assumed that each null hypothesis is that the parameter is equal to 0.
- seed(#) Sets seed to indicate the initial value for the pseudo-random number generator. # can be any integer between 0 and 2^31-1.
- reps(#) Perform # bootstrap replication; default is reps(100). Where possible prefer a larger number of replications for more precise p-values. In IV models, a considerably larger number of replications is highly recommended.
- verbose Requests additional output, including display of the initial (uncorrected) models estimated. This will also result in the generation of a summary output message indicating the number of hypotheses rejected in uncorrected models and when implementing the Romano-Wolf correction, as well as any dependent variables for which the null is rejected in the Romano-Wolf procedure.
- strata(varlist) specifies the variables identifying strata. If strata() is specified, bootstrap samples are selected within each stratum when forming the resampled null distributions.
- cluster(varlist) specifies the variables identifying resampling clusters. If cluster() is specified, the sample drawn when forming the resampled null distributions is a bootstrap sample of clusters. This option does not cluster standard errors in each original regression. If desired, this should be additionally specified using vce(cluster clustvar). It is suggested that these options be used together to ensure that underlying regression models and bootstrap resampling obey the same clustering schemes. If vce(cluster clustvar) is indicated, it is assumed that a clustered bootstrap resample is desired, and cluster() will cluster on the same {cmd clustvar}. If this is not desired, the regcluster() option should be used, which allows for a cluster variable to be passed only to the underlying regressions, or for different cluster variables to be used for the regression, and the bootstrap resamples.
- regcluster(varname) allows for a cluster variable to be passed directly to the regressions used in each test. This option allows for different variables to be used for clustering in the underlying regression (via regcluster()) and the bootstrap resample (via clustvar()), or for a variable to be used to cluster the underlying regression, but not cluster the bootstrap resample procedure.
- onesided(string) Indicates that p-values based on one-sided tests should be calculated. Unless specified, p-values based on two-sided tests are provided, corresponding to the null that each parameter is equal to 0 (or the values indicated in nulls()). In onesided( string), string must be either "positive", in which case the null is that each parameter is greater than or equal to 0, or "negative" in which case the null is that each parameter is less than or equal to 0.
- iv(varlist) only necessary when method(ivregress) is specified. The instrumental variables for the treatment variable of interest should be specified in iv(). At least as many instruments as endogenous variables must be included.
- otherendog(varlist) If more than one endogenous variable is required in ivregress models, additional endogenous variables can be included using this option. By default, when ivregress is specified it is assumed that the variable specified in indepvar(varname) is an endogenous variable which must be instrumented. If this is the case, the variable should not be entered again in otherendog( varlist).
- indepexog If ivregress is specified, but indepvar(varname) is an exogenous variable, indepexog should be indicated. In this case all endogenous variables must be specified in otherendog(varlist) and all instruments must be specified in iv(varlist).
- bl(string) Allows for the inclusion of baseline measures of the dependent variable as controls in each model. If desired, these variables should be created with some suffix, and the suffix should be included in the bl() option. For example, if outcome variables are called y1, y2 and y3, variables y1_bl, y2_bl and y3_bl should be created with baseline values, and bl(_bl) should be specified.
- noplusone Calculate the Resampled and Romano-Wolf adjusted p-values without adding one to the numerator and denominator.
- nodots Suppress replication dots in bootstrap resamples.
- holm Along with standard output, additionally provide p-values corresponding to the Holm multiple hypothesis correction.
- graph Requests that a graph be produced showing the Romano-Wolf null distribution corresponding to each variable examined.
- varlabels Name panels on the graph of null distributions using their variable labels rather than their variable names.
- other options Any additional options which correspond to the baseline regression model. All options permitted by the indicated method are allowed.
- nobootstraps Indicates that bootstrap replications do not need to be estimated by the rwolf command. In this case, each variable indicated in depvars must consist of M bootstrap realizations of the statistic of interest corresponding to each of the multiple baseline models. Additionally, for each variable indicated in depvars, the corresponding standard errors for each of the M bootstrap replicates should be stored as another variable, and these variables should be indicated as stdests(varlist). Finally, the original estimates corresponding to each model in the full sample should be provided in pointestimates(numlist), and the original standard errors should be provided in stderrs( numlist). This option may not be specified if indepvar() and method() are specified. For all standard implementations based on regression models, indepvar() and method() should be preferred.
- pointestimates(numlist) Provides the estimated statistics of interest in the full sample corresponding to each of the depvars indicated in the command. These estimates must be provided in the same order as the depvars are specified. This option may not be specified if indepvar() and method() are specified. For all standard implementations based on regression models, indepvar() and method() should be preferred.
- stderrs(numlist) Provides the estimated standard errors for each estimated statistic in the full sample. These estimates must be provided in the same order as the depvars are specified. This option may not be specified if indepvar() and method() are specified. For all standard implementations based on regression models, indepvar() and method() should be preferred.
- stdests(varlist) Contains variables consisting of estimated standard errors from each of the M resampled replications. These standard errors should correspond to the resampled estimates listed as each depvar and must be provided in the same order as the depvars are specified. This option may not be specified if indepvar() and method() are specified. For all standard implementations based on regression models, indepvar() and method() should be preferred.
- nullimposed Indicates that resamples are centered around the null, rather than the original estimate. This option is generally only used when permutations rather than bootstrap resamples are performed.
sysuse auto
. rwolf headroom turn price rep78, indepvar(weight) controls(trunk mpg) reps(250) seed(121316)
Bootstrap replications (250). This may take some time.
----+--- 1 ---+--- 2 ---+--- 3 ---+--- 4 ---+--- 5
.................................................. 50
.................................................. 100
.................................................. 150
.................................................. 200
.................................................. 250
Romano-Wolf step-down adjusted p-values
Independent variable: weight
Outcome variables: headroom turn price rep78
Number of resamples: 250
------------------------------------------------------------------------------
Outcome Variable | Model p-value Resample p-value Romano-Wolf p-value
--------------------+---------------------------------------------------------
headroom | 0.6719 0.6574 0.6574
turn | 0.0000 0.0040 0.0040
price | 0.0075 0.0359 0.0478
rep78 | 0.0998 0.0797 0.1474
------------------------------------------------------------------------------D. Clarke, J. P. Romano, and M. Wolf. The Romano–Wolf Multiple-hypothesis Correction in Stata. The Stata Journal, 20(4):812–843, 2020.
J. P. Romano and M. Wolf. Stepwise Multiple Testing as Formalized Data Snooping. Econometrica, 73(4): 1237–1282, 2005a.
J. P. Romano and M. Wolf. Exact and Approximate Stepdown Methods for Multiple Hypothesis Testing. Journal of the American Statistical Association, 100(469):94–108, 2005b.
J. P. Romano and M. Wolf. Efficient computation of adjusted p-values for resampling-based stepdown multiple testing. Statistics and Probability Letters, 113:38–40, 2016.