GRAPHICAL AND NUMERICAL DESCRIPTIONS OF DATA

 

Objectives:

(1)   summarizing datasets graphically and numerically

(2)   learn some new STATA commands

 

STATA commands that you will find helpful today and in the future:

 

graph varname1, box

dotplot varname1

tabulate varname1

chist varname1

graph varname1, bin(#)

stem varname1

summarize varname1 varname2, detail

infile varname1 varname2 using filename.raw

use filename.dta

 

Activity 1:  Captopril

Let’s look again at the distribution of blood pressures before and after taking the drug captopril as a blood-pressure treatment (15 patients were given the same dose of the drug).  Use the data from http://www.stat.ucla.edu/~rgould/datasets/bloodpressure.raw

 

Recall that there are two variables in this dataset.  The first variable represents the systolic blood pressure before the drug (varname = before), the second variable is the systolic blood pressure after the drug (varname = after).

 

1.  Would you classify these variables as quantitative or qualitative?

 

Let’s look at the distribution of each variable in graphical terms using dotplots.

Type

dotplot before after

 

The two groups appear side-by-side.  If two observations have the same values, then STATA places the dots beside each other.

 

2. What are the smallest and largest values in each distribution?  What would you guestimate for the sample average and median of each distribution?

 

3.  Does the dotplot provide evidence that the drug is effective?  Why?

 

4.  Now try stem and leaf plots.

Type

stem before

 

Let’s describe the distribution of each variable in numeric terms.  To compute measures of center and spread, type

 

            summarize before after, detail

 

4.  What are the median and the sample average of each distribution?  Were your guestimates close?

 

Activity 2: 

Now use graphical and numerical techniques for describing the distribution of the quantitative variables in http://www.stat.ucla.edu/~rgould/datasets/m12s00.dta

 

5.  Identify each variable as either quantitative or qualitative.

 

For each quantitative variable, create a histogram, dotplot, and a stem and leaf plot.  See what happens when you change the number of intervals in the histogram. 

 

Type

graph varname1, bin(5)             then change 5 to 10, 11, 12, 13, 14, 15

 

6.  Describe each distribution in terms of symmetry, number of modes, and whether or not you think there are any outliers. 

 

7.  What are the mode, median, and sample average for the distribution of each variable.  On the basis of this numeric information, describe the shape of the distribution.  To compute the mode, type

 

tabulate varname1

 

8.  The distribution of “height” looks a little unusual.  What might explain the shape of this distribution?