CCPR Computing Services Workshop 2: Stata October 20, 2004

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2. 2 Outline Converting Data between Statistical Packages Stata � Basic Commands Command Syntax Abbreviations Missing Values Combining Data Using do-files Getting Help Updating Stata

3. 3 Converting Data �  Windows Stat/Transfer SAS, Stata, S-Plus, SPSS, Excel, and more Windows interface Enter �in� data and �out� data Enter info on other tabs as necessary Check results!

4. 4 Converting Data �  Unix Stat/Transfer From within stat-transfer invoke stat/transfer (specific to Unix machine) at stat/transfer prompt, enter: copy datfile1.ext1 datfile2.ext2 datfile1.ext1 = original file, datfile2.ext2 = new file From Unix prompt st datfile1.ext1 datfile2.ext2 (replace st with local Stat/Transfer invocation) See manual for more info and options Check results!

5. 5 Converting Data � DBMS/Copy DBMS/Copy for Unix (without xwindows) From Unix prompt dbmsnox indatfile.ext1 outdatfile.ext2 ext1 and ext2 are �pseudo� extensions spsswin = SPSS for Windows Stata7 = Stata 7 sas7sun = SAS for Unix v7 ssdsun = SAS for Unix v6 Example � windows spss to stata7 dbmsnox mydat.spsswin mydat.stata7 Check results!

6. 6 Converting Data See ATS website for transferring files between SAS, Stata, and SPSS http://www.ats.ucla.edu/stat/sas/faq/convert_pkg.htm

7. 7 Stata - Getting Started Windows: Programs > Stata8 Command Window: enter commands Results Window Other: review, variables, do-editor Unix: Interactive Stata commands and results show in same window Batch Stata nice +10 stata �b do myjob.do

8. 8 Basic Commands Handout 1 (green) Reading raw data insheet, input, infix, infile Using/saving a Stata dataset use, webuse save

9. 9 Basic Commands, cont. Describing data describe Summarize codebook inspect Listing data list Tables of statistics table tab1 varlist (one-way tabulation of variables) tab2 varlist (two-way tabulations of variables)

10. 10 Basic Commands, cont. Changing data drop keep generate encode var, generate newvar recode replace

11. 11 Basic Commands, cont. Labeling data label variable label define label values label list

12. 12 A few other commands compress - saves data more efficiently reshape � long/wide sort/ gsort order rename

13. 13 Stata Syntax Basic command syntax: [by varlist:] command [varlist] [= exp] [if exp] [in range] [weighttype=weight] [, options] Brackets = optional portions Italics = user specified

14. 14 Complete syntax [by varlist:] command [varlist] [= exp] [if exp] [in range] [weighttype=weight] [, options] Example 1 (webuse union) Stata Command: .summarize Result: Summarizes all dataset variables (_all) Stata Syntax, cont.

15. 15 Complete syntax [by varlist:] command [varlist] [= exp] [if exp] [in range] [weighttype=weight] [, options] Example 2 (webuse union) Stata command: .summarize age Result: Summarizes variable age Stata Syntax, cont.

16. 16 Complete syntax [by varlist:] command [varlist] [= exp] [if exp] [in range] [weighttype=weight] [, options] Example 3 (webuse union) Stata Command: .summarize age if year >= 80 Result: Summarizes age, includes only observations with year >= 80 Stata Syntax, cont.

17. 17 Complete syntax [by varlist:] command [varlist] [= exp] [if exp] [in range] [weighttype=weight] [, options] Example 4 (webuse union) Stata Command: .summarize age if year >= 80 in 1/100 Result: Summarizes variable age, includes only first 100 obs and only obs with year >= 80 Stata Syntax, cont.

18. 18 Complete syntax [by varlist:] command [varlist] [= exp] [if exp] [in range] [weighttype=weight] [, options] Example 5 (webuse union) Stata Command: .by black: summarize age if year >= 80 Result: Summarizes age separately for different values of black, including only obs for which year >= 80 Stata Syntax, cont.

19. 19 Complete syntax [by varlist:] command [varlist] [= exp] [if exp] [in range] [weighttype=weight] [, options] Example 6 (webuse union) Stata Command: .bysort black: summarize age if year >= 80, detail Result: Detailed summaries of variable age, separated over different values of black, includes only obs with year >= 80 Stata Syntax, cont.

20. 20 Complete syntax [by varlist:] command [varlist] [= exp] [if exp] [in range] [weighttype=weight] [, options] Example 7 (webuse union) Generally [= exp] used with commands generate and replace Stata Commands: .generate agelt30 = age .replace agelt30 = 0 if age < 30 .replace agelt30 = 0 if age >= 30 & age < . Result: Variable agelt30 set equal to 1, 0, or missing Stata Syntax, cont.

21. 21 Complete syntax [by varlist:] command [varlist] [= exp] [if exp] [in range] [weighttype=weight] [, options] Example 8 Stata Command: .summarize race [pweight=final_wt] Results: Summarizes variable race accounting for probability weight called final_wt. Note: There are four different types of weights in Stata�be careful. Stata Syntax, cont.

22. 22 Abbreviations in Stata Abbreviating command, option, and variable names shortest uniquely identifying name is sufficient Example: Variables in use = make, price, mpg Stata command, not abbreviated: .summarize make price Stata command, abbreviated: .su ma p Exceptions describe (d), list (l), and some others Commands that change/delete Functions implemented by ado-files

23. 23 Missing Values in Stata 8 Stata 8 27 representations of numerical �missing� ., .a, .b, � , .z Relational comparisons Biggest number < . < .a < .b < � < .z Mathematical functions missing + nonmissing = missing String missing = Empty quote: ��

24. 24 Missing Values in Stata - Pitfalls Pitfall #1 Stata7 vs. Stata8 missing values: Pitfall #2 Do NOT: .replace weightlt200 = 0 if weight >= 200 INSTEAD: .replace weightlt200 = 0 if weight >= 200 & weight < .

25. 25 Combining Data Append vs. Merge Append � same variables, different observations Merge - same or related observations, different variables Appending data in Stata Handout 2

26. 26 Combining Data- merge and joinby Demonstrate with two sample datasets: Neighborhood and County samples One-to-one merge Handout 3 One-to-many merge � use match merge Handout 4 Many-to-many merge � use joinby Handout 5

27. 27 Combining Data Variable _merge (generated by merge and joinby) �update� option also includes _merge=4,5 �update� changes default action when matched observation has missing values in master and non-missing in �using� data Pitfalls Pitfall_merge1 handout 6 Pitfall_merge2 handout 7

28. 28 Do-files What is a do-file? Stata commands can be executed interactively or via a do-file A do-file is a text file containing commands that can be read by Stata Handouts are do-files Stata command .do dofilename.do

29. 29 Do-files Why use a do-file? Documentation Communication Reproduce interactive session? Interactive vs. do-files Record EVERYTHING to recreate results in your do-file!!

30. 30 Do-files > Header, Version Control Header Include in do-files � name, project, project location, date, purpose, inputs, outputs, special instructions Version Control include version at top of do-file Why?

31. 31 Do-file > End of Line Character Commands requiring multiple lines delimit ; This command tells Stata to read semi-colons as the end-of-line character instead of the carriage return Comment out the carriage return with /* at the end of line and */ at the beginning of next Comment out the carriage return with ///

32. 32 Do-files > End of line Character Example 1: #delimit #delimit ; keep firstname lastname birth death age weight height; #delimit cr Example 2: /* */ keep firstname lastname birth /* */ age weight height Example 3: /// keep firstname lastname birth /// age weight height

33. 33 Do-files > Comments Comments Lines beginning with * will be ignored Words between /* and */ will be ignored (spanning multiple lines ok) Words between // and end of line will be ignored Words between /// and beginning of next line will be ignored (one way to spread command over two lines)

34. 34 Do-files > Comments Comments - example

35. 35 Saving output Work in do-files and log your sessions! log using filename replace, append log close Output choices: *.log file - ASCII file *.smcl file - nicer format for viewing and printing in Stata

36. 36 Basic Commands, cont. Graphs are not saved in log files Use �saving� option of graph commands saving(graph.ext) Export current graph: graph export graph.ext Ex: graph export graph.eps Supported formats: .ps, .eps, .wmf, .emf .pict

37. 37 Getting Help in Stata help command_name abbreviated version of manual search search keywords, local search keywords, net search keywords, all findit keywords same as search keywords, all Search Stata Listserver and Stata FAQ

38. 38 Stata Resources www.stata.com > Resources and Support Search Stata Listserver Search Stata (FAQ) Stata Journal (SJ) articles for subscribers programs free Stata Technical Bulletin (STB) replaced with the Stata Journal Articles available for purchase, programs free Courses (for fee)

39. 39 Updating Stata help update update all