Lab 1: Introduction to Stata

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Materials

• driving_2004.dta

Objectives¹

By the end of this tutorial you should be able to complete the following tasks in Stata:

• Identify key areas of the Stata interface

• Open a data file

• Summarize and tabulate data

• Create and save a log file

• Open, view, and save a data file

• How to get help with Stata

If you need more help, check out Stata Resources.

General command structure

do {something} ... with {variable(s) x}...if {something is true..}, options

Key commands

Logic statements

• tab bac10 if gdl==1 & sl70plus == 0

  • Tabulates the variable bac10 but only if gdl equals one and sl70plus equals 0

• tab bac10 if year >=2000

  • Tabulates the variable bac10 for the years 2000, 2001, 2002, etc.

• tab bac10 if year !=2000:

  • Tabulates the variable bac10 for every year but 2000

• tab bac10 if year < 2008 & year > 2005

  • Tabulates the variable bac10 2006 and 2007

• tab bac10 if year < 2008 | year > 2005

  • Tabulates the variable bac10 is less than 2008 OR greater than 2005 (all years!)

Also: you can use parentheses to group terms appropriately. For example, if you want to tabulate states where the speed limit is 55 or 65 AND the blood alcohol limit is 0.10, then this is wrong:

tab state if sl55 == 1 | sl65 == 1 & bac10 == 1

But this is correct!

tab state if (sl55 == 1 | sl65 == 1) & bac10 == 1

Thanks, parentheses!

Hey, Stata. It’s nice to meet you

Start by opening Stata. You should have a window that looks something like this (on a PC):

You should now have the Stata window open. There is a set of pull down menus as well as 4 smaller windows: Review, Variables, Results, and Command.

Also especially helpful are the following buttons:

Log files

If you want to record anything that you do in a Stata session so that you can look at results or commands later, you need to open a log-file. A log-file is simply a record of all the commands you enter into Stata and the output from those commands. The key is to make sure you have a log file open at the beginning of a Stata session, and to close it once you have finished, and before you close Stata.

There are three ways you can open a log file:

1. Go to the FILE dropdown menu, choose Log, choose Begin. You should see a “Begin Logging Stata Output” dialog box. Browse to a directory where you can store your log file and type in the following file name in the File Name space: lab1.log

2. Click on the log icon at the top of the Stata workspace (right of the print button). When you click on the log button, the “Begin Logging Stata Output” dialog box pops up. Name your log file as above.

3. You can open a log file by typing the following in the Stata command window: log using lab1.log, replace

   The , replace is optional. If you add it as an option, your new file will overwrite your old one. Or, you can add , append to add it to the bottom of your old log file.

Tip: Use extension .log, NOT the default .smcl. This will make it easier for you to edit, cut and paste your log in any text editor.

Now that you have a log file open, we can start our STATA session.

Opening data files

Stata data files end with the extension .dta, and they can only be read by Stata. You can import text files and excel files into Stata, and you can export .dta files into text files or Excel files, but we’ll cover this later.

There are three ways to open a data file:

1. Outside Stata, double click on the data file you want to open

2. Use the FILE/OPEN drop down menu in Stata and open the data set that you copied into your folder. Note that in the command window, the use command appears. We’ll use that one later.

3. Type use filename.dta, clear into the command window within Stata

Download driving_2004.dta and open it. It is a file of driving laws, vehicle accidents, and fatalities in the United States in 2004.

You should now see the list of variables appear in the Variables window, with the variable name, variable label, and some other information.

Looking at data

Let’s take a more detailed look at the variables in the dataset.

In the command window type: describe

At the top of the output you will see some overall features of the file, including the number of variables. Below that you will see a list of every variable, including the variable name, the “storage type” (byte, float, int, etc.) and the variable label. If you see –more– at the bottom of your screen, press the space bar to continue scrolling.²

To learn more about the variables and the organization of the data, use the browse command. Type: browse (or click on the “browse” button).

Another approach is to add a variable list to the browse command. Type the following:

browse year sl70plus bac10 bac08 gdl

Again, note that you can also double click on the variable names so you don’t have to type them all!

This command directs you to a spreadsheet inside Stata where the data appears. This looks a lot like an Excel spreadsheet!

Note the following:

• Each observation appears on a separate row of the spreadsheet, which represents data from a certain year and a certain state. For example the first row is for state 1 (Alabama) in 1980. If you move along the row, you can see other characteristics about Alabama in 1980.

• Each variable appears in a separate column, and the name of the variable is at the column heading.

How many observations are there? What type of data set is this?

Examining variables

Let’s look at the variables that are included in the data set. There is an efficient way to find the names of variables you are interested in. Suppose you are interested in a variable related to alcohol laws. Type in:

lookfor alcohol

This will give you a list of all the variables that have “alcohol” in either their variable name or variable label. In this case, two variables appear - bac10 and bac08.

You can also experiment with all possible combinations of the col, row, and cell options, and add the nofreq option to suppress the number of observations. Use help for details:

help tab

When you are analyzing variables, you will want to think carefully about whether you should be looking at row percentages, column percentages, or cell percentages.

Lab Exercise 1

First, work through the above steps. Then, work through the 7 questions below.

What do I submit?

1. Your written up answers to exercise questions (1) - (7). This can be typed or written out then scanned (or photographed), in any reasonable format
2. A log file that contains the results from the steps prior to the exercise and the exercise itself.

Questions

1. How many states have graduated drivers license laws (GDLs)? How many states have speed limits of 70 mph or higher (including no speed limit)?

2. What percentage of states with GDLs and with low speed limits (below 70 mph) have blood-alcohol limits of 0.10 (the more lenient level)? Note that some states have blood-alcohol limit for a fraction of a year. If so, consider having a limit of 0.10 in place for part of the year as having a limit

3. What is the mean fatality rate per 100 million miles across all states? What is the standard deviation?

4. What was the fatality rate (deaths per 100 million miles) in Vermont? (Vermont is state 46)

5. Generate a variable $Y$ equal to one if a state has a fatality rate per 100 million miles that is above the mean, and zero otherwise. What is $E(Y)$?

6. Write a joint probability distribution table for the following two random variables: $X$, a random variable equal to one if a state has a speed limit of 70 or greater and zero otherwise (see sl70plus), and $Y$, the random variable developed in the previous part.

7. Look up the command correlate in the help files: What is the correlation coefficient between nighttime fatalities per 100,000 population and weekend accidents per 100,000 population? Why might this correlation be so strong?