Stata - Welcome to CCPR — California Center for Population
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Transcript Stata - Welcome to CCPR — California Center for Population
CCPR Computing Services
Workshop: Introduction to Stata
June, 2006
1
Outline
Stata
Command Syntax
Basic Commands
Abbreviations
Missing Values
Combining Data
Using do-files
Basic programming
Special Topics
Getting Help
Updating Stata
2
Stata Syntax
Basic command syntax:
[by varlist:]
command [varlist] [= exp] [if exp] [in range] [weighttype=weight] [, options]
Brackets = optional portions
Italics = user specified
3
Stata Syntax, cont.
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.
4
Stata Syntax, cont.
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
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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
List string variables in current dataset
ds
ds, has(type string)
Save current dataset
save tempdata/myauto
8
Generating New Variables
Create new variable = headroom squared
generate headroom2 = headroom^2
Generate numeric from string variable
encode 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 < .
10
Generating New Variables, cont.
Generate categorical version of numerical
variable: Method 2
generate priceCat2 = price
recode priceCat2 (min/5000 = 1) (5000/10000=2) (10000/max=3)
Compare price, priceCat, and priceCat2
table 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 format:
Long format:
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 -> Long:
reshape long uniqueschool author, i(year session order) j(count)
Long -> Wide:
reshape wide author, i(year session order) j(count)
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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
Relational comparisons
Biggest number < . < .a < .b < … < .z
Mathematical functions
27 representations of numerical “missing”
., .a, .b, … , .z
missing + nonmissing = missing
String missing =
Empty quote: “”
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Missing Values in Stata - Pitfalls
Pitfall #1
Missing values changed after Stata7:
Stata 7
Stata 8 and later
varname != .
varname < .
varname == .
varname >= .
Pitfall #2
Do NOT:
.replace weightlt200 = 0 if weight >= 200
INSTEAD:
.replace weightlt200 = 0 if weight >= 200 & weight < .
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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
18
Combining Data- merge and joinby
Demonstrate with two sample datasets:
One-to-one merge
onetoone.do
One-to-many merge – use match merge
Neighborhood and County samples
onetomany.do
Many-to-many merge – use joinby
manytomany.do
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Combining Data
Variable _merge (generated by merge and joinby)
_merge
Observation in master data
Observation in “using” data
1
Yes
No
2
No
Yes
3
Yes
Yes
Pitfalls
pitfall_merge1.do: Merging unsorted data
pitfall_merge2.do : many-to-many using merge instead of
joinby
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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 ;
Comment out the carriage return with
This command tells Stata to read semi-colons as the
end-of-line character instead of the carriage return
/* 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
26
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
27
Saving Output, cont.
Graphs are not saved in log files
Use “saving” option of graph commands
Export current graph:
saving(graph.ext)
graph export graph.ext
Ex: graph export graph.eps
Supported formats:
.ps, .eps, .wmf, .emf .pict
28
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
29
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’ "
}
30
Example using forvalues, display
*see samplePrograms.do, runCount
forvalues counter = 1/10 {
display `counter'
}
forvalues counter = 0(2)10 {
display `counter'
}
31
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'
}
32
Example – if/else
*see samplePrograms.do, runIfElse
foreach 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"
}
}
33
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]+$")
34
Special Topic: accessing data in another
database
odbc list
odbc query testStata
odbc query testStata
odbc desc "Summary2006$“
odbc load year type session order author1
author2, table("Summary2006$")
dsn("testStata")
35
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
search
search keywords, local
search keywords, net
search keywords, all
findit keywords
abbreviated version of manual
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)
Stata Technical Bulletin (STB)
articles for subscribers
programs free
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
40