Stata

1
CCPR Computing ServicesWorkshop 2 StataOctober
20, 2004
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Outline

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

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
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!

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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!

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Converting Data

• See ATS website for transferring files between
  SAS, Stata, and SPSS
• http//www.ats.ucla.edu/stat/sas/faq/convert_pkg.h
  tm

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Stata - Getting Started

• Windows Programs gt 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

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Basic Commands

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

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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)

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Basic Commands, cont.

• Changing data
• drop
• keep
• generate
• encode var, generate newvar
• recode
• replace

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Basic Commands, cont.

• Labeling data
• label variable
• label define
• label values
• label list

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A few other commands

• compress - saves data more efficiently
• reshape long/wide
• sort/ gsort
• order
• rename

13
Stata Syntax

• Basic command syntax
• by varlist
• command varlist exp if exp in range
  weighttypeweight , options
• Brackets optional portions
• Italics user specified

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Stata Syntax, cont.

• Complete syntax
• by varlist
• command varlist exp if exp in range
  weighttypeweight , options
• Example 1 (webuse union)
• Stata Command
• .summarize
• Result Summarizes all dataset variables (_all)

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Stata Syntax, cont.

• Complete syntax
• by varlist
• command varlist exp if exp in range
  weighttypeweight , options
• Example 2 (webuse union)
• Stata command
• .summarize age
• Result Summarizes variable age

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Stata Syntax, cont.

• Complete syntax
• by varlist
• command varlist exp if exp in range
  weighttypeweight , options
• Example 3 (webuse union)
• Stata Command
• .summarize age if year gt 80
• Result
• Summarizes age, includes only observations with
  year gt 80

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Stata Syntax, cont.

• Complete syntax
• by varlist
• command varlist exp if exp in range
  weighttypeweight , options
• Example 4 (webuse union)
• Stata Command
• .summarize age if year gt 80 in 1/100
• Result
• Summarizes variable age, includes only first 100
  obs and only obs with year gt 80

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Stata Syntax, cont.

• Complete syntax
• by varlist
• command varlist exp if exp in range
  weighttypeweight , options
• Example 5 (webuse union)
• Stata Command
• .by black summarize age if year gt 80
• Result
• Summarizes age separately for different values of
  black, including only obs for which year gt 80

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Stata Syntax, cont.

• Complete syntax
• by varlist
• command varlist exp if exp in range
  weighttypeweight , options
• Example 6 (webuse union)
• Stata Command
• .bysort black summarize age if year gt 80,
  detail
• Result
• Detailed summaries of variable age, separated
  over different values of black, includes only obs
  with year gt 80

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Stata Syntax, cont.

• Complete syntax
• by varlist
• command varlist exp if exp in range
  weighttypeweight , options
• Example 7 (webuse union)
• Generally exp used with commands generate and
  replace
• Stata Commands
• .generate agelt30 age
• .replace agelt30 0 if age lt 30
• .replace agelt30 0 if age gt 30 age lt .
• Result
• Variable agelt30 set equal to 1, 0, or missing

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Stata Syntax, cont.

• Complete syntax
• by varlist
• command varlist exp if exp in range
  weighttypeweight , options
• Example 8
• Stata Command
• .summarize race pweightfinal_wt
• Results
• Summarizes variable race accounting for
  probability weight called final_wt.
• Note
• There are four different types of weights in
  Statabe careful.

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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

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Missing Values in Stata 8

• Stata 8
• 27 representations of numerical missing
• ., .a, .b, , .z
• Relational comparisons
• Biggest number lt . lt .a lt .b lt lt .z
• Mathematical functions
• missing nonmissing missing
• String missing
• Empty quote

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Missing Values in Stata - Pitfalls

• Pitfall 1
• Stata7 vs. Stata8 missing values
• Pitfall 2
• Do NOT
• .replace weightlt200 0 if weight gt 200
• INSTEAD
• .replace weightlt200 0 if weight gt 200
  weight lt .

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Combining Data

• Append vs. Merge
• Append same variables, different observations
• Merge - same or related observations, different
  variables
• Appending data in Stata
• Handout 2

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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

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Combining Data

• Variable _merge (generated by merge and joinby)
• update option also includes _merge4,5
• 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

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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

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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!!

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Do-files gt 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?

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Do-file gt 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 ///

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Do-files gt 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

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Do-files gt 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)

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Do-files gt Comments

• Comments - example

SAMPLE EXCERPT OF STATA DO-FILE This line will
be ignored by Stata. use mydata.dta / These
words will be ignored / do myjob.do //The
remainder of this line will be ignored. keep
age race sex ///The remainder of this line will
be ignored, including return first_name height
weight last_name /This line continuation of the
last line
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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

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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

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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

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Stata Resources

• www.stata.com gt 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)

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Updating Stata

• help update
• update all