Introduction To Stata

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Introduction to StataChristopher F BaumFaculty Micro Resource CenterBoston CollegeAugust 2011Christopher F Baum (Boston College FMRC)Introduction to StataAugust 20111 / 157

Strengths of StataWhat is Stata?Overview of the Stata environmentStata is a full-featured statistical programming language for Windows,Mac OS X, Unix and Linux. It can be considered a “stat package,” likeSAS, SPSS, RATS, or eViews.Stata is available in several versions: Stata/IC (the standard version),Stata/SE (an extended version) and Stata/MP (for multiprocessing).The major difference between the versions is the number of variablesallowed in memory, which is limited to 2,047 in standard Stata/IC, butcan be much larger in Stata/SE or Stata/MP. The number ofobservations in any version is limited only by memory.Christopher F Baum (Boston College FMRC)Introduction to StataAugust 20112 / 157

Strengths of StataWhat is Stata?Stata/SE relaxes the Stata/IC constraint on the number of variables,while Stata/MP is the multiprocessor version, capable of utilizing 2, 4,8. processors available on a single computer. Stata/IC will meet mostusers’ needs; if you have access to Stata/SE or Stata/MP, you can usethat program to create a subset of a large survey dataset with fewerthan 2,047 variables. Stata runs on all 64-bit operating systems, andcan access larger datasets on a 64-bit OS, which can address a largermemory space.All versions of Stata provide the full set of features and commands:there are no special add-ons or ‘toolboxes’. Each copy of Stataincludes a complete set of manuals (over 6,000 pages) in PDF format,hyperlinked to the on-line help.Christopher F Baum (Boston College FMRC)Introduction to StataAugust 20113 / 157

Strengths of StataWhat is Stata?A Stata license may be used on any machine which supports Stata(Mac OS X, Windows, Linux): there are no machine-specific licensesfor Stata versions 11 or 12. You may install Stata on a home and officemachine, as long as they are not used concurrently. Licenses can beeither annual or perpetual.Stata works differently than some other packages in requiring that theentire dataset to be analyzed must reside in memory. This brings aconsiderable speed advantage, but implies that you may need moreRAM (memory) on your computer. There are 32-bit and 64-bit versionsof Stata, with the major difference being the amount of memory thatthe operating system can allocate to Stata (or any other application).Christopher F Baum (Boston College FMRC)Introduction to StataAugust 20114 / 157

Strengths of StataWhat is Stata?In some cases, the memory requirement may be of little concern.Stata is capable of holding data very efficiently, and even a quitesizable dataset (e.g., more than one million observations on 20–30variables) may only require 500 Mb or so. You should take advantageof the compress command, which will check to see whether eachvariable may be held in fewer bytes than its current allocation.For instance, indicator (dummy) variables and categorical variableswith fewer than 100 levels can be held in a single byte, and integersless than 32,000 can be held in two bytes: see help datatypes fordetails. By default, floating-point numbers are held in four bytes,providing about seven digits of accuracy. Some other statisticalprograms routinely use eight bytes to store all numeric variables.Christopher F Baum (Boston College FMRC)Introduction to StataAugust 20115 / 157

Strengths of StataWhat is Stata?The memory available to Stata may be considerably less than theamount of RAM installed on your computer. If you have a 32-bitoperating system, it does not matter that you might have 4 Gb or moreof RAM installed; Stata will only be able to access about 1 Gb,depending on other processes’ demands.To make most effective use of Stata with large datasets, use acomputer with a 64-bit operating system. Stata will automatically installa 64-bit version of the program if it is supported by the operatingsystem. All Linux, Unix and Mac OS X computers today come with64-bit operating systems.Christopher F Baum (Boston College FMRC)Introduction to StataAugust 20116 / 157

Strengths of StataPortabilityStata is eminently portable, and its developers are committed tocross-platform compatibility. Stata runs the same way on Windows,Mac OS X, Unix, and Linux systems. The only platform-specificaspects of using Stata are those related to native operating systemcommands: e.g. is the file to be /statadata/myfile.dtaPerhaps unique among statistical packages, Stata’s binary data filesmay be freely copied from one platform to any other, or even accessedover the Internet from any machine that runs Stata. You may storeStata’s binary datafiles on a webserver (HTTP server) and open themon any machine with access to that server.Christopher F Baum (Boston College FMRC)Introduction to StataAugust 20117 / 157

Strengths of StataStata’s user interfaceStata’s user interfaceStata has traditionally been a command-line-driven package thatoperates in a graphical (windowed) environment. Stata version 11(released June 2009) and version 12 (released July 2011) contains agraphical user interface (GUI) for command entry via menus anddialogs. Stata may also be used in a command-line environment on ashared system (e.g., a Unix server) if you do not have a graphicalinterface to that system.A major advantage of Stata’s GUI system is that you always have theoption of reviewing the command that has been entered in Stata’sReview window. Thus, you may examine the syntax, revise it in theCommand window and resubmit it. You may find that this is a moreefficient way of using the program than relying wholly on dialogs.Christopher F Baum (Boston College FMRC)Introduction to StataAugust 20118 / 157

Strengths of StataStata’s user interfaceStata (version 11): default screen appearance:Christopher F Baum (Boston College FMRC)Introduction to StataAugust 20119 / 157

Strengths of StataStata’s user interfaceThe Toolbar contains icons that allow you to Open and Save files, Printresults, control Logs, and manipulate windows. Some very importanttools allow you to open the Do-File Editor, the Data Editor and the DataBrowser.The Data Editor and Data Browser present you with a spreadsheet-likeview of the data, no matter how large your dataset may be. TheDo-File editor, as we will discuss, allows you to construct a file of Statacommands, or “do-file”, and execute it in whole or in part from theeditor.Christopher F Baum (Boston College FMRC)Introduction to StataAugust 201110 / 157

Strengths of StataStata’s user interfaceThe Toolbar also contains an important piece of information: theCurrent Working Directory, or cwd. In the screenshot, it is listed as/Users/Baum/Documents/ as I am working on a Mac OS X (Unix)laptop. The cwd is the directory to which any files created in your Statasession will be saved. Likewise, if you try to open a file and give itsname alone, it is assumed to reside in the cwd. If it is in anotherlocation, you must change the cwd [File Change Working Directory]or qualify its name with the directory in which it resides.You generally will not want to locate or save files in the default cwd. Acommon strategy is to set up a directory for each project or task in aconvenient location in the filesystem and change the cwd to thatdirectory when working on that task. This can be automated in ado-file with the cd command.Christopher F Baum (Boston College FMRC)Introduction to StataAugust 201111 / 157

Strengths of StataStata’s user interfaceThere are four windows in the default interface: the Review, Results,Command and Variables window. You may alter the appearance of anywindow in the GUI using the Preferences General dialog, and makethose changes on a temporary or permanent basis.As you might expect, you may type commands in the Commandwindow. You may only enter one command in that window, so youshould not try pasting a list of several commands. When a command isexecuted—with or without error—it appears in the Review window, andthe results of the command (or an error message) appears in theResults window. You may click on any command in the Review windowand it will reappear in the Command window, where it may be editedand resubmitted.Christopher F Baum (Boston College FMRC)Introduction to StataAugust 201112 / 157

Strengths of StataStata’s user interfaceOnce you have loaded data into the program, the Variables window willbe populated with information on each variable. That informationincludes the variable name, its label (if any), its type and its format.This is a subset of information available from the describe command.Let’s look at the interface after I have loaded one of the datasetsprovided with Stata, uslifeexp, with the sysuse command andgiven the describe and summarize commands:Christopher F Baum (Boston College FMRC)Introduction to StataAugust 201113 / 157

Strengths of StataChristopher F Baum (Boston College FMRC)Stata’s user interfaceIntroduction to StataAugust 201114 / 157

Strengths of StataStata’s user interfaceNotice that the three commands are listed in the Review window. If anyhad failed, the rc column would contain a nonzero number, in red,indicating the error code. The Variables window contains the list ofvariables and their labels. The Results window shows the effects ofsummarize: for each variable, the number of observations, theirmean, standard deviation, minimum and maximum. If there were anystring variables in the dataset, they would be listed as having zeroobservations.Try it out: type the commandssysuse uslifeexpdescribesummarizeTake note of an important design feature of Stata. If you do not saywhat to describe or summarize, Stata assumes you want to performthose commands for every variable in memory, as shown here. As weshall see, this design principle holds throughout the program.Christopher F Baum (Boston College FMRC)Introduction to StataAugust 201115 / 157

Strengths of StataUsing the Do-File EditorWe may also write a do-file in the do-file editor and execute it. TheDo-File Editor icon on the Toolbar brings up a window in which we maytype those same three commands, as well as a few more:sysuse uslifeexpdescribesummarizenotessummarize le if year 1950summarize le if year 1950After typing those commands into the window, the rightmost icon, withtooltip Do, may be used to execute them.Christopher F Baum (Boston College FMRC)Introduction to StataAugust 201116 / 157

Strengths of StataChristopher F Baum (Boston College FMRC)Using the Do-File EditorIntroduction to StataAugust 201117 / 157

Strengths of StataUsing the Do-File EditorIn this do-file, I have included the notes command to display the notessaved with the dataset, and included two comment lines. There areseveral styles of comments available. In this style, anything on a linefollowing a double slash (//) is ignored.You may use the other icons in the Do-File Editor window to save yourdo-file (to the cwd or elsewhere), print it, or edit its contents. You mayalso select a portion of the file with the mouse and execute only thosecommands. Note that the tooltip changes to Do Selected Lines.Christopher F Baum (Boston College FMRC)Introduction to StataAugust 201118 / 157

Strengths of StataChristopher F Baum (Boston College FMRC)Using the Do-File EditorIntroduction to StataAugust 201119 / 157

Strengths of StataUsing the Do-File EditorTry it out: use the Do-File Editor to open the do-file S1.1.do, and runthe file.Try selecting only those last four lines and run those commands.Christopher F Baum (Boston College FMRC)Introduction to StataAugust 201120 / 157

Strengths of StataThe help systemThe rightmost menu on the menu bar is labeled Help. From that menu,you can search for help on any command or feature. The HelpBrowser, which opens in a Viewer window, provides hyperlinks, in blue,to additional help pages. At the foot of each help screen, there arehyperlinks to the full manuals, which are accessible in PDF format.The links will take you directly to the appropriate page of the manual.You may also search for help at the command line with helpcommand. But what if you don’t know the exact command name?Then you may use search or its expanded version, findit, each ofwhich may be followed by one or several words.Christopher F Baum (Boston College FMRC)Introduction to StataAugust 201121 / 157

Strengths of StataThe help systemResults from search are presented in the Results window, whilefindit results will appear in a Viewer window. Those commands willpresent results from a keyword database and from the Internet: forinstance, FAQs from the Stata website, articles in the Stata Journaland Stata Technical Bulletin, and downloadable routines from the SSCArchive (about which more later) and user sites.Try it out: when you are connected to the Internet, type the commandsearch baum, auand then tryfindit baumNote the hyperlinks that appear on URLs for the books and journalarticles, and on the individual software packages (e.g., st0030 3,archlm).Christopher F Baum (Boston College FMRC)Introduction to StataAugust 201122 / 157

Strengths of StataData ManipulationStata is advertised as having three major strengths:data manipulationstatisticsgraphicsStata is an excellent tool for data manipulation: moving data fromexternal sources into the program, cleaning it up, generating newvariables, generating summary data sets, merging data sets andchecking for merge errors, collapsing cross–section time-series dataon either of its dimensions, reshaping data sets from “long” to “wide”,and so on. In this context, Stata is an excellent program for answeringad hoc questions about any aspect of the data.Christopher F Baum (Boston College FMRC)Introduction to StataAugust 201123 / 157

Strengths of StataStatisticsIn terms of statistics, Stata provides all of the standard univariate,bivariate and multivariate statistical tools, from descriptive statisticsand t-tests through one-, two- and N-way ANOVA, regression, principalcomponents, and the like. Stata’s regression capabilities arefull-featured, including regression diagnostics, prediction, robustestimation of standard errors, instrumental variables and two-stageleast squares, seemingly unrelated regressions, vectorautoregressions and error correction models, etc. It has a verypowerful set of techniques for the analysis of limited dependentvariables: logit, probit, ordered logit and probit, multinomial logit, andthe like.Christopher F Baum (Boston College FMRC)Introduction to StataAugust 201124 / 157

Strengths of StataStatisticsStata’s breadth and depth really shines in terms of its specializedstatistical capabilities. These include environments for time-serieseconometrics (ARCH, ARIMA, ARFIMA, VAR, VEC), model simulationand bootstrapping, maximum likelihood estimation, GMM, andnonlinear least squares. Families of commands provide the leadingtechniques utilized in each of several categories:“xt” commands for cross-section/time-series or panel(longitudinal) data“sem” commands for structural equation modeling“svy” commands for the handling of survey data with complexsampling designs“st” commands for the handling of survival-time data with durationmodelsChristopher F Baum (Boston College FMRC)Introduction to StataAugust 201125 / 157

Strengths of StataGraphicsStata graphics are excellent tools for exploratory data analysis, andcan produce high-quality 2-D publication-quality graphics in severaldozen different forms. Every aspect of graphics may be programmedand customized, and new graph types and graph “schemes” are beingcontinuously developed. The programmability of graphics implies thata number of similar graphs may be generated without any “pointingand clicking” to alter aspects of the graphs. Stata 12 provides supportfor contour plots and ‘heatmaps’.Christopher F Baum (Boston College FMRC)Introduction to StataAugust 201126 / 157

Strengths of StataStata’s update facilityStata’s update facilityOne of Stata’s great strengths is that it can be updated over theInternet. Stata is actually a web browser, so it may contact Stata’s webserver and enquire whether there are more recent versions of eitherStata’s executable (the kernel) or the ado-files. This enables Stata’sdevelopers to distribute bug fixes, enhancements to existingcommands, and even entirely new commands during the lifetime of agiven major release (including ‘dot-releases’ such as Stata 11.1).Updates during the life of the version you own are free. You need onlyhave a licensed copy of Stata and access to the Internet (which maybe by proxy server) to check for and, if desired, download the updates.Christopher F Baum (Boston College FMRC)Introduction to StataAugust 201127 / 157

Strengths of StataExtensibilityExtensibility of official StataAnother advantage of the command-line driven environment involvesextensibility: the continual expansion of Stata’s capabilities. Acommand, to Stata, is a verb instructing the program to perform someaction.Commands may be “built in” commands—those elements sofrequently used that they have been coded into the “Stata kernel.” Arelatively small fraction of the total number of official Stata commandsare built in, but they are used very heavily.Christopher F Baum (Boston College FMRC)Introduction to StataAugust 201128 / 157

Strengths of StataExtensibilityThe vast majority of Stata commands are written in Stata’s ownprogramming language–the “ado-file” language. If a command is notbuilt in to the Stata kernel, Stata searches for it along the adopath.Like the PATH in Unix, Linux or DOS, the adopath indicates theseveral directories in which an ado-file might be located. This impliesthat the “official” Stata commands are not limited to those coded intothe kernel. Try it out: give the adopath command in Stata.If Stata’s developers tomorrow wrote a new command named “foobar”,they would make two files available on their web site: foobar.ado(the ado-file code) and foobar.sthlp (the associated help file). Bothare ordinary, readable ASCII text files. These files should be producedin a text editor, not a word processing program.Christopher F Baum (Boston College FMRC)Introduction to StataAugust 201129 / 157

Strengths of StataExtensibilityThe importance of this program design goes far beyond the limits ofofficial Stata. Since the adopath includes both Stata directories andother directories on your hard disk (or on a server’s filesystem), youmay acquire new Stata commands from a number of web sites. TheStata Journal (SJ), a quarterly refereed journal, is the primary methodfor distributing user contributions. Between 1991 and 2001, the StataTechnical Bulletin played this role, and a complete set of issues of theSTB are available on line at the Stata website.The SJ is a subscription publication (articles more than three years oldfreely downloadable), but the ado- and sthlp-files may be freelydownloaded from Stata’s web site. The Stata help commandaccesses help on all installed commands; the Stata command finditwill locate commands that have been documented in the STB and theSJ, and with one click you may install them in your version of Stata.Help for these commands will then be available in your own copy.Christopher F Baum (Boston College FMRC)Introduction to StataAugust 201130 / 157

Strengths of StataExtensibilityUser extensibility: the SSC archiveBut this is only the beginning. Stata users worldwide participate in theStataList listserv, and when a user has written and documented a newgeneral-purpose command to extend Stata functionality, theyannounce it on the StataList listserv (to which you may freelysubscribe: see Stata’s web site).Since September 1997, all items posted to StataList (over 1,300)have been placed in the Boston College Statistical SoftwareComponents (SSC) Archive in RePEc (Research Papers inEconomics), available from IDEAS (http://ideas.repec.org) andEconPapers (http://econpapers.repec.org).Christopher F Baum (Boston College FMRC)Introduction to StataAugust 201131 / 157

Strengths of StataExtensibilityAny component in the SSC archive may be readily inspected with aweb browser, using IDEAS’ or EconPapers’ search functions, and ifdesired you may install it with one command from the archive fromwithin Stata. For instance, if you know there is a module in the archivenamed mvsumm, you could use ssc describe mvsumm to learnmore about it, and ssc install mvsumm to install it if you wish.Anything in the archive can be accessed via Stata’s ssc command:thus ssc describe mvsumm will locate this module, and make itpossible to install it with one click.Windows users should not attempt to download the materials from aweb browser; it won’t work.Try it out: when you are connected to the Internet, typessc describe mvsummssc install mvsummhelp mvsummChristopher F Baum (Boston College FMRC)Introduction to StataAugust 201132 / 157

Strengths of StataExtensibilityThe command ssc new lists, in the Stata Viewer, all SSC packagesthat have been added or modified in the last month. You may click ontheir names for full details. The command ssc hot reports on themost popular packages on the SSC Archive.The Stata command adoupdate checks to see whether all packagesyou have downloaded and installed from the SSC archive, the StataJournal, or other user-maintained net from. sites are up to date.adoupdate alone will provide a list of packages that have beenupdated. You may then use adoupdate, update to refresh yourcopies of those packages, or specify which packages are to beupdated.Christopher F Baum (Boston College FMRC)Introduction to StataAugust 201133 / 157

Strengths of StataExtensibilityThe importance of all this is that Stata is infinitely extensible. Anyado-file on your adopath is a full-fledged Stata command. Stata’scapabilities thus extend far beyond the official, supported featuresdescribed in the Stata manual to a vast array of additional tools.Since the current directory is on the adopath, if you create an ado-filehello.ado:program define hellodisplay "Stata says hello!"endexitStata will now respond to the command hello. It’s that easy. Try it out!Christopher F Baum (Boston College FMRC)Introduction to StataAugust 201134 / 157

Strengths of StataAvailability, Cost, and SupportFor members of the Boston College community, Stata is availablethrough ITS’ applications server, http://apps.bc.edu. Afterdownloading client software from this site, you may connect to theapps server from any BC-activated computer and run Stata in awindow on your computer. It is actually running the Windows version ofStata/SE 11.2, but the interface and commands is almost identical toStata for Mac OS X or Stata for Linux. Up to 50 users may accessStata on the apps server simultaneously. Results from your analysismay be stored on MyFiles, as the m: disk is automatically mapped toyour account on appstorage.bc.edu, accessible from any webbrowser with authentication. If you are working from off campus, youmust use set up VPN on your computer; seehttp://www.bc.edu/help for details.Christopher F Baum (Boston College FMRC)Introduction to StataAugust 201135 / 157

Strengths of StataAvailability, Cost, and SupportIf you would like your own copy of Stata, it is quite inexpensive. Thevendor’s GradPlan program makes the full version of Stata version 12software available to BC faculty and students for 98.00 (one-yearlicense) or 179.00 (perpetual license). This includes the full set ofmanuals in PDF format, hyperlinked to Stata’s help system.The “Small Stata” version is available to students for 49.00 for aone-year license. It contains all of Stata’s commands, but can onlyhandle a limited number of observations and variables (thus notrecommended for Ph.D. students or Senior Honors Thesis students).GradPlan orders are made direct to Stata, with delivery fromon-campus inventory.Christopher F Baum (Boston College FMRC)Introduction to StataAugust 201136 / 157

Strengths of StataAvailability, Cost, and SupportStata is very well supported by telephone and email technical support,as well as the more informal support provided by other users onStataList, the Stata listserv. The manuals are useful—particularly theUser’s Guide—but full details of the command syntax are availableonline, and in hypertext form in the GUI environment, with hyperlinks tothe appropriate pages of the full documentation set of over a dozenmanuals. The command findit keyword can also be used to locateStata materials, including descriptions of built-in commands, StataFAQs, and hundreds of user-written routines.Christopher F Baum (Boston College FMRC)Introduction to StataAugust 201137 / 157

Working with the command lineBut why should I type commands?But before we discuss the specifics to back up these claims, let’sconsider a meta-issue: why would you want to learn how to use acommand-line-driven package? Isn’t that ever so 20th century?Stata may be used in an interactive mode, and those learning thepackage may wish to make use of the menu system. But when youexecute a command from a pull-down menu, it records the commandthat you could have typed in the Review window, and thus you maylearn that with experience you could type that command (or modify itand resubmit it) more quickly than by use of the menus.Let us consider a couple of reasons why a command-line-drivenpackage makes for an effective and efficient research strategy.Christopher F Baum (Boston College FMRC)Introduction to StataAugust 201138 / 157

Working with the command lineAdvantage: ReproducibilityReproducibilityFirst, the important issue of reproducibility. If you are conductingscientific research, you must be able to reproduce your results. Ideally,anyone with your programs and data should be able to do so withoutyour assistance. If you cannot produce such reproducible researchfindings, it can be argued that you are not following the scientificmethod, nor is your work conforming to ethical standards of research.A thorough discussion of this issue is covered in the html.Christopher F Baum (Boston College FMRC)Introduction to StataAugust 201139 / 157

Working with the command lineAdvantage: ReproducibilityIn a computer program where all actions are point and click, such as aspreadsheet, who can say how you arrived at a certain set of results?Unless every step of your transformations of the data can be retraced,how can you find exactly how the sample you are employing differsfrom the raw data? A command-driven program is capable of this levelof reproducibility, we should all instill this level of rigor in our researchpractices.Reproducibility also makes it very easy to perform an alternateanalysis of a particular model. What would happen if we added thisinteraction, or introduced this additional variable, or decided to handlezero values as missing? Even if many steps have been taken since thebasic model was specified, it is easy to go back and produce avariation on the analysis if all the work is represented by a series ofprograms.Christopher F Baum (Boston College FMRC)Introduction to StataAugust 201140 / 157

Working with the command lineAdvantage: TransportabilityTransportabilityStata binary files may be easily transformed into SPSS or SAS fileswith the third-party application Stat/Transfer. Stat/Transfer is availablefor Windows and Mac OS X systems as well as on various Unixsystems on campus. Personal copies of Stat/Transfer version 11(which handles Stata versions 6, 7, 8, 9, 10, 11 and 12 datafiles) areavailable at a discounted academic rate of 69.00 through the StataGradPlan.Stat/Transfer can also transfer SAS, SPSS and many other file formatsinto Stata format, without loss of variable labels, value labels, and thelike. It can also be used to create a manageable subset of a very largeStata file (such as those produced from survey data) by selecting onlythe variables you need. It is a very useful tool.Christopher F Baum (Boston College FMRC)Introduction to StataAugust 201141 / 157

Working with the command lineProgrammability of tasksProgrammability of tasksStata may be used in an interactive mode, and those learning thepackage may wish to make use of the menu system. But when youexecute a command from a pull-down menu, it records the commandthat you could have typed in the Review window, and thus you maylearn that with experience you could type that command (or modify itand resubmit it) more quickly than by use of the menus.Stata makes reproducibility very easy through a log facility, the abilityto generate a command log (containing only the commands you haveentered), and the do-file editor which allows you to easily enter,execute and save sequences of commands, or program fragments.Christopher F Baum (Boston College FMRC)Introduction to StataAugust 201142 / 157

Working with the command lineProgrammability of tasksGoing one step further, if you use the do-file editor to create asequence of commands, you may save that do-file and reuse ittomorrow, or use it as the starting point for a similar set of datamanagement or statistical operations. Working in this way promotesreproducibility, which makes it very easy to perform an alternateanalysis of a particular model. Even if many steps have been takensince the basic model was specified, it is easy to go back and producea variation on the analysis if all the work is represented by a series ofprograms.One of the implications of the concern for reproducible work: avoidaltering data in a non-auditable environment such as a spreadsheet.Rather, you should transfer external data into the Stata environment asearly as possible in the process of analysis, and only make permanentchanges to the data with do-files that can give you an audit trail ofevery change made to the data.Christopher F Baum (Boston College FMRC)Introduction

Stata is available in several versions: Stata/IC (the standard version), Stata/SE (an extended version) and Stata/MP (for multiprocessing). The major difference between the versions is the number of variables allowed in memory, which is limited to 2,047 in standard Stata/IC, but can be much larger in Stata/SE or Stata/MP. The number of

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