Stata

1
CCPR Computing ServicesWorkshop Introduction to
StataJune, 2006
2
Outline

• Stata
• Command Syntax
• Basic Commands
• Abbreviations
• Missing Values
• Combining Data
• Using do-files
• Basic programming
• Special Topics
• Getting Help
• Updating Stata

3
Stata Syntax

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

4
Stata Syntax, cont.

• Complete syntax
• by varlist
• command varlist exp if exp in range
  weighttypeweight , options
• Example 1 (webuse union)
• Stata Command
• .bysort black summarize age if year gt 80,
  detail
• Results
• Summarizes age separately for different values of
  black, including only observations for which year
  gt 80, includes extra detail.

5
Stata Syntax, cont.

• Complete syntax
• by varlist
• command varlist exp if exp in range
  weighttypeweight , options
• Example 2 (webuse union)
• Stata Commands
• .generate agelt30 age
• .replace agelt30 1 if age lt 30
• .replace agelt30 0 if age gt 30 age lt .
• Result
• Variable agelt30 set equal to 1, 0, or missing
• Generally exp used with commands generate and
  replace

6
Basic Commands Load auto data and look at
some vars

• Load data from Statas website
• webuse auto.dta
• Look at dataset
• describe
• Summarize some variables
• codebook make headroom, header
• inspect weight length

7
Basic Commands Load auto data and look at
some vars

• Look at first and last observation
• list make price mpg rep78 if _n1
• list make price mpg rep78 if _n_N
• Summarize a variable in a table
• table foreign
• table foreign, c(mean mpg sd mpg)

8
Keep/Save a Subset of the Data

• Keep a subset of the variables in memory
• keep make headroom trunk weight length
• List variables in current dataset
• ds
• List string variables in current dataset
• ds, has(type string)
• Save current dataset
• save tempdata/myauto

9
Generating New Variables

• Create new variable headroom squared
• generate headroom2 headroom2
• Generate numeric from string variable
• encode make, generate(makeNum)
• list make makeNum in 1/5
• Cant tell its numeric, but look at storage
  type in describe
• describe make makeNum

10
Generating New Variables, cont.

• Create categorical variable from continuous
  variable
• price is integer-valued with minimum 3291 and
  max 15906
• Generate categorical version - Method 1
• generate priceCat 0
• replace priceCat 1 if price lt 5000
• replace priceCat 2 if price gt 5000 price lt
  10000
• replace priceCat 3 if price gt 10000 price lt .

11
Generating New Variables, cont.

• Generate categorical version of numerical
  variable Method 2
• generate priceCat2 price
• recode priceCat2 (min/5000 1) (5000/100002)
  (10000/max3)
• Compare price, priceCat, and priceCat2
• table price priceCat
• table priceCat priceCat2

12
Variable Labels and Value Labels

• Create a description for a variable
• label variable priceCat Categorical price"
• Create labels to represent variable values
• label define priceCatLabels 1 cheap 2 mid-range 3
  expensive
• label values priceCat priceCatLabels
• View results
• describe
• list price priceCat in 1/10

13
Reshape
Wide format Long format

• Wide -gt Long
• reshape long uniqueschool author, i(year session
  order) j(count)
• Long -gt Wide
• reshape wide author, i(year session order)
  j(count)

14
A few other commands

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

15
Abbreviations in Stata

• Abbreviating command, option, and variable names
• shortest uniquely identifying name is sufficient
• Example
• Assume three variables are in use make, price,
  mpg
• UN-abbreviated Stata command
• .summarize make price
• Abbreviated Stata command
• .su ma p
• Exceptions
• describe (d), list (l), and some others
• Commands that change/delete
• Functions implemented by ado-files

16
Missing Values in Stata 8 and 9

• Stata 8 and later versions
• 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

17
Missing Values in Stata - Pitfalls

• Pitfall 1
• Missing values changed after Stata7
• Pitfall 2
• Do NOT
• .replace weightlt200 0 if weight gt 200
• INSTEAD
• .replace weightlt200 0 if weight gt 200
  weight lt .

18
Combining Data

• Append vs. Merge
• Append two datasets with same variables,
  different observations
• Merge two datasets with same or related
  observations, different variables
• Appending data in Stata
• Example append.do

19
Combining Data- merge and joinby

• Demonstrate with two sample datasets
• Neighborhood and County samples
• One-to-one merge
• onetoone.do
• One-to-many merge use match merge
• onetomany.do
• Many-to-many merge use joinby
• manytomany.do

20
Combining Data

• Variable _merge (generated by merge and joinby)
• Pitfalls
• pitfall_merge1.do Merging unsorted data
• pitfall_merge2.do many-to-many using merge
  instead of joinby

21
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
• Running a do-file within Stata
• .do dofilename.do

22
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!

23
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?
• Example
• Under version 7, ..a.b..z

24
Do-files gt Comments

• Comments
• Lines beginning with will be ignored
• Words between // and end of line will be ignored
• Spanning commands over two lines
• Words between / and / will be ignored,
  including end of line character
• Words between /// and beginning of next line will
  be ignored

25
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 ///

26
Do-files gt Examples

• webuse auto, clear
• this is a comment
• delimit
• summarize price mpg rep78
• headroom trunk weight
• delimit cr
• summarize price mpg rep78 headroom trunk weight
  //this is a comment
• summarize price mpg rep78 ///
• headroom trunk weight
• summarize price mpg rep78 /
• / headroom trunk weight

27
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

28
Saving Output, 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

29
Example using local macro

• . local mypath "C\Documents and
  Settings\MyStata"
• . display mypath'
• C\Documents invalid name
• r(198)
• . display C\Documents and Settings\MyStata
• C\Documents invalid name
• r(198)
• . display "mypath'"
• C\Documents and Settings\MyStata

30
Example foreach, return, display

• see samplePrograms.do, runLoop
• foreach var of varlist tenure-lnwage
• quietly summarize var'
• local varmean r(mean)
• display "Variable var' has mean varmean "

31
Example using forvalues, display

• see samplePrograms.do, runCount
• forvalues counter 1/10
• display counter'
• forvalues counter 0(2)10
• display counter'

32
Example forvalues, generating random variables

• see samplePrograms.do, runRandomGen
• forvalues j 1/3
• generate xj' uniform()
• generate yj' invnormal(uniform())
• foreach x of varlist x1-x3 y1-y3
• summarize x'

33
Example if/else

• see samplePrograms.do, runIfElse
• foreach var of varlist tenure-ln_wage
• quietly summarize var'
• local varmean r(mean)
• if varmean' gt 10
• display "var' has mean greater than 10"
• else
• display "var' has mean less than 10"

34
Special Topic regular expressions

• webuse auto
• List all values of make starting with a capital
  and containing an additional capital
• list make if regexm(make, "A-Z.A-Z.")
• AND ending in a number
• list make if regexm(make, "A-Z.A-Z.0-9
  ")

35
Special Topic accessing data in another database

• odbc list
• odbc query testStata
• odbc query testStata
• odbc desc "Summary2006
• odbc load year type session order author1
  author2, table("Summary2006") dsn("testStata")

36
Special Topic Exporting results using outreg

• User-written program called outreg
• From within Stata, type findit outreg
• Very simple!!
• Basically add one line of code after each
  regression to export results
• For an example of code, see http//www.ats.ucla.ed
  u/stat/stata/faq/outreg.htm

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
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)

39
Updating Stata

• help update
• update all

40

• Questions/feedback