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Stata Class Notes 3.0
Exploring Data

1.0 Stata commands in this unit

2.0 Demonstration and Explanation

We will begin by loading hs0.dta, a dataset saved in Stata's format that we encountered in the Entering Data unit. Stata data files end with the dta extension. Stata data files are loaded into memory using the use command. If you don't have the floppy disk you can load the data over the Internet using this command: 

use http://www.ats.ucla.edu/stat/stata/notes3/hs0, clear

use hs0, clear 

Before we start our statistical exploration we will look at the data using the describe, list and codebook commands. Note that the variable prgtype is a string variable.

describe
list
list gender-read
codebook 

Next, we will open a log file which will save all of the commands and the output in a text file.

log using unit2, text replace 

The basic descriptive statistics command in Stata is summarize. Along with summarize, we also show the tabstat and table commands for displaying descriptive statistics within groups.

summarize
summarize read math science write
display 9.48^2                               /* note: variance is the sd (9.48) squared */
summarize write, detail
sum write if read>=60                   /* note: sum is abbreviation of summarize */
sum write if prgtype=="academic"
sum write in 1/40

tabulate prgtype, summarize(read)
tabulate prgtype, summarize(write)
tabulate prgtype, summarize(math)

tabstat read write math, by(prgtype) stat(n mean sd)

tabstat write, stat(n mean sd p25 p50 p75) by(prgtype)

Next, let's use some graphics commands to look at our data. We will begin with stem which generates an ASCII stem-and-leaf plot. We will also use the graph command with the hist (histogram) and box (boxplot) options. We also show the kdensity command which produces a smoothed density plot.

stem write
stem write, lines(2)

histogram write, normal
histogram write, normal start(30) width(5)

kdensity write, normal
kdensity write, normal width(5)  /* a smoother kdensity plot */
kdensity math, normal

graph box write
graph box write, over(prgtype) 

The tabulate command can produce one-way or two-way frequency tables. The tab1 command is a convenience command to produce multiple one-way frequency tables. The hist command is used to display histograms for categorical variables.

histogram ses
histogram ses, discrete
tabulate ses
tab write       /* note: tab is abbreviation of tabulate */
tab1 gender schtyp prgtype

tab prgtype ses
tab prgtype ses, all 

There are two commands to create correlation matrices, correlate which uses listwise deletion of missing data and pwcorr which uses pairwise deletion. The general purpose graph command produces scatter plots using the twoway option and an scatterplot matrix using the matrix option. The jitter option is used to spread apart identical observations.

correlate write read science
pwcorr write read science, obs
scatter write read
scatter write read, jitter(2)
graph matrix read science write, half 

We have completed all of the analyses in this unit, so it is time to close the log file.

log close 

Now, let's see what is in the log file. We will do this using the type command.

type unit2.log 

3.0 For More Information

• Statistics with Stata 9
  • Chapters 3 and 4
• Stata Learning Modules
  • Descriptive information and statistics
  • Using if with Stata commands

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