1. Day 1: Getting Started Department of Economics
Trinity College Dublin, Ireland

2. Getting Started Contents: An Introduction to Stata Creating do files
Creating log files
Commands to get you started
Opening, sorting and merging data files
Cleaning data
Descriptive Statistics

3. 1. An Introduction to Stata
Starting Stata:
Start  Programs  Stata  Stata SE 8
Description of package:
Menu bar
Tool bar
Command Window
Results Window
Variables Window
Review Window
Data Browser
Data Editor
Do file editor

4. 2. Creating do-files
Projects are easiest to manage if you record all of your work in a ‘do-file’
Typing doedit or clicking the do editor button opens an empty do-file. A do file is saved with extension .do
To run a series of commands saved in a do-file under filename.do, type:
do filename.do
To run only a selection of command lines in a do-file, highlight them and click the “run” button inside the open do file.

5. 3. Creating log-files
A log-file records all results returned by (a series of) STATA commands
To open a log-file (this can be done at any point in a series of commands):
log using poverty.smcl or poverty.log
Note: using ‘.log’ allows you open log file in Word
If the log file already exists, you can overwrite it by:
log using poverty.log, replace
Or you can add more information to the end of an existing log file by:
log using poverty.log, append
To close a log-file:
log close
To temporarily suspend writing to a log or to resume it:
log off
log on
see help log for more details

6. 4. Commands to get you started
clear removes all data from memory
set memory instructs Stata to create space to store data
no; data in memory would be lost
op sys refuses to provide memory
set maxvar maximum number of variables
set matsize maximum number of variables that can be included in a model
When to use these commands:
no room to add more observations
no room to add more variables
query memory or memory displays current memory and data settings
Use , permanently to add changes permanently
cd “file location” directs STATA to the path on your PC where your data are stored

7. 4. Commands to get you started
STATA usually displays results by screenfulls. A key needs to be touched to make the rest of a command continue. By typing:
set more off
in the beginning of the do-file, you avoid this. Set more off makes the programme run screen after screen. Turn it on again by:
set more on
“More” can also be set to equal a certain number x, the program will then run window after window, pausing each time for x seconds between them

8. 4. Commands to get you started
STATA will stop automatically if a command is entered incorrectly
In some cases you may prefer for STATA to continue, even if an error is made. For example, if you ask STATA to start a new log file and there is already a log file open it will stop running the do-file
To instruct STATA to continue with the do-file use the command capture
So instead of
log close
Use
capture log close

9. 4. Commands to get you started
Commenting
To keep track of what is contained in the do-file, start a do-file by assigning a title, the date it was produced or changed, by whom it was created, which datasets are used, and other relevant information which will make it easier for the do-file to be used later by yourself or by someone else.
Comments written between /* comment */ will be ignored by STATA and not executed. A whole line can be ignored by typing an asterisk in front:
* the next command shows averages of rural consumption:
sum consumption if rural==1 /* urban cons. is shown in descriptive table */

10. EXERCISE ONE Open the do-file “Day1.do” stored on your desktop
Write your name and the date in the title box
Clear all data from Stata’s memory
Set Stata up so that 200 megabytes of RAM is allocated to store the data
Set the path to “C:\Data”
Open a new log file called “Day1.log”

11. 5. Opening, sorting and organising data files
Open a Stata dataset: use filename.dta [, clear]
Save a Stata file: save filename.dta [, replace]
browse: allows you view the data
describe [varlist]: lists and describes variables
Numeric variables
String variables
codebook [varlist]: stats and other information
inspect [varlist]: more statistical information
count [varlist]: counts the number of observations that satisfy a specified condition
sort [varlist]: sorts data in ascending order of varlist
gsort [varlist]: same as sort but allows you specify whether in ascending or descending order (use ‘+’ or ‘-’)
erase datafile.dta: permanently erases a data file from your computer

12. 5. Opening, sorting and organising data files
keep [varlist]: keeps the variables listed and drops all other variables in the data set
drop [varlist]: drops the variables listed from the data set
if varname==#
’=’ means assign a value
’==’ means check whether a variable has a specific value

13. 5. Opening, sorting and organising data files
Merging Data:
Both files have to be in Stata format
Both have to have at least one of the same variables the identifies each observation
This variable should be sorted in the same way
merge identifier list using filename
Use tab _merge to check merged data
_merge=1 if the observation is only present in the master dataset
_merge=2 if the observation is only present in the incoming dataset
_merge=3 if the observation is present in both datasets

14. 5. Opening, sorting and organising data files
Appending Data:
Allows more observations to be added to the same dataset
append using filename

15. EXERCISE TWO Open the data file “Day1.dta”
‘count’ the number of observations in the data set
Use the ‘describe’ command to review the data in memory
Use the ‘codebook’ command to review the data in memory
Use the ‘inspect’ command to produce detailed information on the structure of the variables in your dataset, i.e., the number of negative and zero values, the number of missing values etc.
Open the individual data file “individual.dta”
Keep observations on the household head
Sort by household and save as “temp1.dta”
Open “Day1.dta” and merge with “temp1.dta”
Tabulate the variable ‘_merge’ and then drop it
Erase the data file “temp1.dta”
Set the sample (samp_report==1)
Sort by household and save changes to “Day1.dta”

16. 6. Cleaning data label variable varname Assigns label to varname
label values varname valuename Assigns values to varname For example: categorical variables
label define valuename codes Defines the value labels
label data “……” Labels the actual data
label list Lists all the labels stored

17. 6. Cleaning data
generate variable=value generates a variable with a specified value for each observation.
generate var2=var1 replicates a variable already in the dataset
(for editing later for example)
generate var2=var1-var0 creates a new variable that is some function of other variables in the dataset

18. 6. Cleaning data
replace variable=value changes the value of a variable for each observation
Note: usually specified with an ‘if’ command to selectively change variable values
replace variable=value if var1==10
This would mean that you would only replace the value of the variable in cases where var1 takes a value of 10
Other operators can also be used such as, ‘>’ or ‘<’ or ‘>=’ or ‘<=’
OR and AND commands can also be used
The AND command is given by ‘&’:
replace variable=value if var1<=10 & var2>5
The OR command is given by ‘|’:
replace variable=value if var1==10 | var1==11
’!=’ means not equal to!

19. 7. Descriptive Statistics
summarize varlist: Summary statistics for all variables in varlist
, detail: displays additional stats
By Options:
by varname, sort: summarize varlist
If already sorted by varname no need for ’sort’ option
bysort varname: summarize varlist
summarize varlist if varname==#
Saved Results:
return list: Lists the results saved by Stata
’=’ means assign a value
’==’ means check whether a variable has a specific value

20. 7. Descriptive Statistics
Tables of Summary Statistics
tabstat varname: Displays table of summary stats
Options:
allows you specify statistics: stats(mean sd)
by: for two way tables
missing: show number of missing observations

21. 7. Descriptive Statistics
Tabulate
tab varname or tab1 varnames: Gives frequencies for varname(s)
tab2 varname1 varname 2: Gives two way frequencies
Options:
nolabel for numeric values instead of value labels
missing to display missing values
plot for graphical comparison
summarize for summary statistics

22. 8. Graphs graph twoway scatterplots, line plots etc.
graph matrix scatterplot matrices
graph bar bar charts
graph pie pie charts

23. 8. Graphs Bar charts: graph bar (stat) varname, over(catvar)
draws bar chart of a statistic, stat, of varname for a categorical variable given by catvar.
Default for stat is the mean
Search help graph bar for all options
Pie charts:
graph pie varname
Saving/Printing graphs:
graph save graphname filename.gph
graph print

24. 9. Applying weights
Most surveys have one or more of the following design characteristics:
sampling or probability weights
stratification
clustering
Data are assumed to be representative:
for the whole population they are drawn from
for certain groups (regions or other than geographically determined groups)
Not taking into account the design of the survey could entail that the results returned by STATA commands such as summarise, mean, regress, … only reflect characteristics of the collected data (sample) but not the larger population they are representative off.
=> inaccurate or biased results

25. 9. Applying weights
Example: some groups which are of particular interest to the researcher or which are of particular ease to collect data for, might have been oversampled.
Oversampling = share in the sample does not reflect true share in the actual population the oversampled are part of.
Oversampling => can cause a difference between the sample statistics (point estimates and/or standard errors) and the population statistics.
In order to retrieve the accurate population statistics from the sample, the exact characteristics of the sample (such as how it was designed, stratified, clustered, how observations were drawn) have to be known and taken into account.
How to make sure that the descriptive statistics we report are describing the population rather than the sample?
“manually” by including weights or cluster options
STATA survey commands

26. 9. Applying weights
Assume we have a simple random sample of n observations drawn from a population of size N, the variable of interest is x, with observations xi running from 1 to n.
In the case of simple random design, the sample mean is the correct estimator of the population mean:
BUT! Most surveys do not use a simple random design but different households have different probabilities of being selected into the sample in which case the sample means will be biased estimators of the population means.
In order to calculate correct statistics, observations need to be reweighted: those that are underrepresented will be weighted up and those that are overrepresented will be weighted down.

27. 9. Applying weights
The weights wi are inversely proportional to the probabilities of being selected at each draw, πi
In a simple random design, the probability of being selected is 1/N and the weights are the same for all observations and equal to N/n.
When the probabilities of being selected differ over households, the weights also differ.
The weights indicate how many population households are represented by the sample household i.
Weights are sometimes also called inflation factors, as they inflate the sample to the population. Summing the weights, gives an estimation of the total population size N.

28. 9. Applying weights
The probability weighted mean is equal to the estimated total divided by the estimated population size:
This can be rewritten as:
Where the weights are normalised to add up to one:

29. 9. Applying weights
So where we fail to correct for the weighting of the sample, we will get biased estimators of the population parameters.
The same holds when calculating statistics for each subgroup of the population such as regions, rural/urban areas, men/women, etc.
In STATA we can correct for weighting by explicitly including the variable that holds the weights between square brackets at the end of a command (see help weight)
command vars [w=weight]
you can also specify the type of weights you want to use:
pweights=probability weights (typically used for samples)
aweights=analytical weights
fweights=frequency weights
command vars [pw=weight]

30. 9. Applying weights Household weights versus population weights.
Often we are interested not in household means but in means per person. For example, from a household level dataset we would like to know the proportion of poor people rather than the proportion of poor households, or we would like to know the proportion of poor people by sex or age.
In this case we would need to use population weights rather than household weights. To arrive at the correct weights, we need to multiply the household weights by the household size. The total of these weights is an estimate of the total population (rather than of the total number of households as described before).

31. EXERCISE THREE
Run the command line in the do-file that creates food expenditure quintiles
Run the command lines in the do-file that labels this variable and assigns value labels
Run the command lines in the do-file that tabulate this variable with and without weights and think about what you observe
A number of dummy variables have to be created for Table 1.1. They are
malehead – the sex of the household head
kinh – ethnicity of the household head
viet – head of household speaks Vietnamese
vietmain – Vietnamese is the main language of the head of household
classpoor – household is classified as poor by the authorities
Create a 0-1 indicator variable for each of these and label each variable as appropriate. A number of prompts and hints are given in the do-file

32. EXERCISE THREE
Replicate Table 1.1 and Figure 1.1 from the 2006 report using the 2008 data (Hint: don’t forget to use weights)
Run the commands in the do-file that create the variables ‘suppchout’ and ‘bornhere’. Pay particular attention to the notes on the difference between these variables in 2008 compared with 2006
Replicate Table 1.2 from the 2006 report using the 2008 data
Run the commands in the do-file that create the variables ‘eduhead’ and ‘profhead’
Replicate Table 1.3 from the 2006 report using the 2008 data
Run the commands in the do-file that create the variable ‘disprimary’
Create the following 0-1 dummy variables:
dislowsec – distance to lower secondary school
disuppsec – distance to upper secondary school
dispcoffice – distance to people’s committee office
Replicate Table 1.4 from the 2006 report using the 2008 data

33. EXERCISE THREE
13. Run the commands in the do-file that create Figures 1.2 and 1.3
What do these figures tell you? How do they compare to 2006?
14. Replicate Figures 1.4 and 1.5 from the 2006 report using the data
15. Close the log file and save your changes to the do-file “Day1.do”