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CCPR Computing ServicesWorkshop: Introduction to

StataJune, 2006

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Outline Stata

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

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Stata Syntax

Basic command syntax:[by varlist:]

command [varlist] [= exp] [if exp] [in range] [weighttype=weight] [, options]

Brackets = optional portions Italics = user specified

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Complete syntax[by varlist:]

command [varlist] [= exp] [if exp] [in range] [weighttype=weight] [, options]

Example 1 (webuse union) Stata Command:

.bysort black: summarize age if year >= 80, detail Results:

Summarizes age separately for different values of black, including only observations for which year >= 80, includes extra detail.

Stata Syntax, cont.

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Complete syntax[by varlist:]

command [varlist] [= exp] [if exp] [in range] [weighttype=weight] [, options] Example 2 (webuse union)

Stata Commands: .generate agelt30 = age.replace agelt30 = 1 if age < 30.replace agelt30 = 0 if age >= 30 & age < .

Result: Variable agelt30 set equal to 1, 0, or missing

Generally [= exp] used with commands generate and replace

Stata Syntax, cont.

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Basic Commands – Load “auto” data and look at some vars Load data from Stata’s website

webuse auto.dta Look at dataset

describe Summarize some variables

codebook make headroom, header

inspect weight length

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Basic Commands – Load “auto” data and look at some vars Look at first and last observation

list make price mpg rep78 if _n==1

list make price mpg rep78 if _n==_N Summarize a variable in a table

table foreign

table foreign, c(mean mpg sd mpg)

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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

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Generating New Variables

Create new variable = headroom squaredgenerate headroom2 = headroom^2

Generate numeric from string variableencode make, generate(makeNum)

list make makeNum in 1/5 Can’t tell it’s numeric, but look at “storage type” in

describe:

describe make makeNum

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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 < 5000

replace priceCat = 2 if price >= 5000 & price < 10000

replace priceCat = 3 if price >= 10000 & price < .

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Generating New Variables, cont. Generate categorical version of numerical

variable: Method 2generate priceCat2 = price

recode priceCat2 (min/5000 = 1) (5000/10000=2) (10000/max=3)

Compare price, priceCat, and priceCat2table price priceCat

table priceCat priceCat2

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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

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Reshape

Wide -> Long: reshape long uniqueschool author, i(year session order) j(count)

Long -> Wide:reshape wide author, i(year session order) j(count)

year Session Order Author1 Author2

2006 P01 3 Biddlecom Bankole

2006 P01 4 Anyara Hinde

2006 P01 5 Amouzou Becker

Year Session Order Author Count

2006 P01 3 Biddlecom 1

2006 P01 3 Bankole 2

2006 P01 4 Anyara 1

2006 P01 4 Hinde 2

2006 P01 5 Amouzou 1

2006 P01 5 Becker 2

Wide format:

Long format:

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A few other commands

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

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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

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Missing Values in Stata 8 and 9 Stata 8 and later versions

27 representations of numerical “missing” ., .a, .b, … , .z

Relational comparisons Biggest number < . < .a < .b < … < .z

Mathematical functions missing + nonmissing = missing

String missing = Empty quote: “”

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Missing Values in Stata - Pitfalls Pitfall #1

Missing values changed after Stata7:

Pitfall #2 Do NOT:

.replace weightlt200 = 0 if weight >= 200

INSTEAD: .replace weightlt200 = 0 if weight >= 200 & weight < .

Stata 7 Stata 8 and later

varname != . varname < .

varname == . varname >= .

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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

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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

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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

_merge Observation in master data Observation in “using” data

1 Yes No

2 No Yes

3 Yes Yes

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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 Running a do-file within Stata

.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 > 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

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Do-files > 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

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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 ///

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Do-files > 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

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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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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

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Example using local macro

. local mypath "C:\Documents and Settings\MyStata"

. display `mypath'C:\Documents invalid namer(198);

. display C:\Documents and Settings\MyStataC:\Documents invalid namer(198);

. display "`mypath'"C:\Documents and Settings\MyStata

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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’ "

}

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Example using forvalues, display*see samplePrograms.do, runCount

forvalues counter = 1/10 {

display `counter'

}

forvalues counter = 0(2)10 {

display `counter'

}

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Example: forvalues, generating random variables*see samplePrograms.do, runRandomGen

forvalues j = 1/3 {

generate x`j' = uniform()

generate y`j' = invnormal(uniform())

}

foreach x of varlist x1-x3 y1-y3 {

summarize `x'

}

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Example – if/else

*see samplePrograms.do, runIfElseforeach var of varlist tenure-ln_wage { quietly summarize `var' local varmean = r(mean) if `varmean' > 10 { display "`var' has mean greater than 10" } else { display "`var' has mean less than 10" }}

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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]+$")

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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")

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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.edu/stat/stata/faq/outreg.htm

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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 > 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

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Questions/feedback