Getting the names, types, formats, lengths, and labels of variables in a SAS data set

After reading my blog post on getting the variable names of a SAS data set, a reader named Robin asked how to get the formats as well.  I asked SAS Technical Support for help, and a consultant named Jerry Leonard provided a beautiful solution using PROC SQL.  Besides the names and formats of the variables, it also gives the types, lengths, and labels.  Here is an example of how to do so with the CLASS data set in the built-in SASHELP library.

* add formats and labels to 3 of the variables in the CLASS data set;
data class;                                                      
       set sashelp.class;                                            
       format 
            age 8.  
            weight height 8.2 
            name $15.;          
       label 
            age = 'Age'
            weight = 'Weight'
            height = 'Height';
run;                                                             
                  

* extract the variable information using PROC SQL; 
proc sql 
       noprint;                                                
       create table class_info as 
       select libname as library, 
              memname as data_set, 
              name as variable_name, 
              type, 
              length, 
              format, 
              label       
       from dictionary.columns                                       
       where libname = 'WORK' and memname = 'CLASS';                     
       /* libname and memname values must be upper case  */         
quit;                                                          
                   
 
* print the resulting table;
proc print 
       data = class_info;                                            
run;

Here is the result of that PROC PRINT step in the Results Viewer.  Notice that it also has the type, length, format, and label of each variable.

Obs library data_set variable_name type length format label
1 WORK CLASS Name char 8 $15.
2 WORK CLASS Sex char 1
3 WORK CLASS Age num 8 8. Age
4 WORK CLASS Height num 8 8.2 Height
5 WORK CLASS Weight num 8 8.2 Weight

Thank you, Jerry, for sharing your tip!

Sorting correlation coefficients by their magnitudes in a SAS macro

Theoretical Background

Many statisticians and data scientists use the correlation coefficient to study the relationship between 2 variables.  For 2 random variables, X and Y, the correlation coefficient between them is defined as their covariance scaled by the product of their standard deviations.  Algebraically, this can be expressed as

\rho_{X, Y} = \frac{Cov(X, Y)}{\sigma_X \sigma_Y} = \frac{E[(X - \mu_X)(Y - \mu_Y)]}{\sigma_X \sigma_Y}.

In real life, you can never know what the true correlation coefficient is, but you can estimate it from data.  The most common estimator for \rho is the Pearson correlation coefficient, which is defined as the sample covariance between X and Y divided by the product of their sample standard deviations.  Since there is a common factor of

\frac{1}{n - 1}

in the numerator and the denominator, they cancel out each other, so the formula simplifies to

r_P = \frac{\sum_{i = 1}^{n}(x_i - \bar{x})(y_i - \bar{y})}{\sqrt{\sum_{i = 1}^{n}(x_i - \bar{x})^2 \sum_{i = 1}^{n}(y_i - \bar{y})^2}} .

 

In predictive modelling, you may want to find the covariates that are most correlated with the response variable before building a regression model.  You can do this by

  1. computing the correlation coefficients
  2. obtaining their absolute values
  3. sorting them by their absolute values.

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How to Extract a String Between 2 Characters in R and SAS

Introduction

I recently needed to work with date values that look like this:

mydate
Jan 23/2
Aug 5/20
Dec 17/2

I wanted to extract the day, and the obvious strategy is to extract the text between the space and the slash.  I needed to think about how to program this carefully in both R and SAS, because

  1. the length of the day could be 1 or 2 characters long
  2. I needed a code that adapted to this varying length from observation to observation
  3. there is no function in either language that is suited exactly for this purpose.

In this tutorial, I will show you how to do this in both R and SAS.  I will write a function in R and a macro program in SAS to do so, and you can use the function and the macro program as you please!

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Using PROC SQL to Find Uncommon Observations Between 2 Data Sets in SAS

A common task in data analysis is to compare 2 data sets and determine the uncommon rows between them.  By “uncommon rows”, I mean rows whose identifier value exists in one data set but not the other. In this tutorial, I will demonstrate how to do so using PROC SQL.

Let’s create 2 data sets.

data dataset1;
      input id $ group $ gender $ age;
      cards;
      111 A Male 11
      111 B Male 11
      222 D Male 12
      333 E Female 13
      666 G Female 14
      999 A Male 15
      999 B Male 15
      999 C Male 15
      ;
run;
data dataset2;
      input id $ group $ gender $ age;
      cards;
      111 A Male 11
      999 C Male 15
      ;
run;

First, let’s identify the observations in dataset1 whose ID variable values don’t exist in dataset2.  I will export this set of observations into a data set called mismatches1, and I will print it for your viewing.  The logic of the code is simple – find the IDs in dataset1 that are not in the IDs in dataset2.  The code “select *” ensures that all columns from dataset1 are used to create the data set in mismatches1.

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Separating Unique and Duplicate Observations Using PROC SORT in SAS 9.3 and Newer Versions

As Fareeza Khurshed commented in my previous blog post, there is a new option in SAS 9.3 and later versions that allows sorting and the identification of duplicates to be done in one step.  My previous trick uses FIRST.variable and LAST.variable to separate the unique observations from the duplicate observations, but that requires sorting the data set first before using the DATA step to do the separation.  If you have SAS 9.3 or a newer version, here is an example of doing it in one step using PROC SORT.

There is a data set called ADOMSG in the SASHELP library that is built into SAS.  It has an identifier called MSGID, and there are duplicates by MSGID.  Let’s create 2 data sets out of SASHELP.ADOMSG:

  • DUPLICATES for storing the duplicate observations
  • SINGLES for storing the unique observations
proc sort
     data = sashelp.adomsg
          out = duplicates
          uniqueout = singles
          nouniquekey;
     by msgid;
run;

Here is the log:

NOTE: There were 459 observations read from the data set SASHELP.ADOMSG.
NOTE: 300 observations with unique key values were deleted.
NOTE: The data set WORK.DUPLICATES has 159 observations and 6 variables.
NOTE: The data set WORK.SINGLES has 300 observations and 6 variables.
NOTE: PROCEDURE SORT used (Total process time):
real time 0.28 seconds
cpu time 0.00 seconds

Note that the number of observations in WORK.DUPLICATES and WORK.SINGLES add to 459, the total number of observations in the original data set.

In addition to Fareeza, I also thank CB for sharing this tip.

Resources for Learning Data Manipulation in R, SAS and Microsoft Excel

I had the great pleasure of speaking to the Department of Statistics and Actuarial Science at Simon Fraser University on last Friday to share my career advice with its students and professors.  I emphasized the importance of learning skills in data manipulation during my presentation, and I want to supplement my presentation by posting some useful resources for this skill.  If you are new to data manipulation, these are good guides for how to get started in R, SAS and Microsoft Excel.

For R, I recommend Winston Chang’s excellent web site, “Cookbook for R“.  It has a specific section on manipulating data; this is a comprehensive list of the basic skills that every data analyst and statistician should learn.

For SAS, I recommend the UCLA statistical computing web page that is adapted from Oliver Schabenberger’s web site.

For Excel, I recommend Excel Easy, a web site that was started at the University of Amsterdam in 2010.  It is a good resource for learning about Excel in general, and there is no background required.  I specifically recommend the “Functions” and “Data Analysis” sections.

A blog called teachr has a good list of Top 10 skills in Excel to learn.

I like to document tips and tricks for R and SAS that I like to use often, especially if I struggled to find them on the Internet.  I encourage you to check them out from time to time, especially in my “Data Analysis” category.

If you have any other favourite resources for learning data manipulation or data analysis, please share them in the comments!

Using PROC SGPLOT to Produce Box Plots with Contrasting Colours in SAS

I previously explained the statistics behind box plots and how to produce them in R in a very detailed tutorial.  I also illustrated how to produce side-by-side box plots with contrasting patterns in R.

Here is an example of how to make box plots in SAS using the VBOX statement in PROC SGPLOT.  I modified the built-in data set SASHELP.CLASS to generate one that suits my needs.

The PROC TEMPLATE statement specifies the contrasting colours to be used for different classes.  I also include code for exportingthe result into a PDF file using ODS PDF.  (I used varying shades of grey to allow the contrast to be shown when printed in black and white.)

boxplots

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Getting a List of the Variable Names of a SAS Data Set

Update on 2017-04-15: I have written a new blog post that obtains the names, types, formats, lengths, and labels of variables in a SAS data set.  This uses PROC SQL instead of PROC CONTENTS.  I thank Robin for suggesting this topic in the comments and Jerry Leonard from SAS Technical Support for teaching me this method.

 

Getting a list of the variable names of a data set is a fairly common and useful task in data analysis and manipulation, but there is actually no procedure or function that will do this directly in SAS.  After some diligent searching on the Internet, I found a few tricks within PROC CONTENTS do accomplish this task.

Here is an example involving the built-in data set SASHELP.CLASS.  The ultimate goal is to create a new data set called “variable_names” that contains the variable names in one column.

The results of PROC CONTENTS can be exported into a new data set.  I will call this data set “data_info”, and it contains just 2 variables that we need – “name” and “varnum“.

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Getting All Duplicates of a SAS Data Set

Introduction

A common task in data manipulation is to obtain all observations that appear multiple times in a data set – in other words, to obtain the duplicates.  It turns out that there is no procedure or function that will directly provide the duplicates of a data set in SAS*.

*Update: As Fareeza Khurshed kindly commented, the NOUNIQUEKEY option in PROC SORT is available in SAS 9.3+ to directly obtain duplicates and unique observations.  I have written a new blog post to illustrate her solution.

The Wrong Way to Obtain Duplicates in SAS

You may think that PROC SORT can accomplish this task with the nodupkey and the dupout options.  However, the output data set from such a procedure does not have the first of each set of duplicates.  Here is an example.

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Extracting the Postal Codes from Addresses of Hospitals in British Columbia – An Exercise in SAS Text Processing

Introduction

In my job as a Biostatistical Analyst at the British Columbia (BC) Cancer Agency in Vancouver, I recently needed to get the postal codes for the hospitals in BC.  I found a data table of the hospitals with their addresses, but I needed to extract the postal codes from the addresses.  In this tutorial, I will show you some text processing techniques in SAS that I used to extract the postal codes from that raw data file.

* This blog post contains information licensed under the Open Government License – British Columbia.

Read the rest of this post to get the SAS code for extracting the postal codes and the final spreadsheet that contains the postal codes of the hospitals in British Columbia!

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Calculating the sum or mean of a numeric (continuous) variable by a group (categorical) variable in SAS

Introduction

A common task in data analysis and statistics is to calculate the sum or mean of a continuous variable.  If that variable can be categorized into 2 or more classes, you may want to get the sum or mean for each class.

This sounds like a simple task, yet I took a surprisingly long time to learn how to do this in SAS and get exactly what I want – a new data with with each category as the identifier and the calculated sum/mean as the value of a second variable.  Here is an example to show you how to do it using PROC MEANS.

Read more to see an example data set and get the SAS code to calculate the sum or mean of a continuous variable by a categorical variable!

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The Chi-Squared Test of Independence – An Example in Both R and SAS

Introduction

The chi-squared test of independence is one of the most basic and common hypothesis tests in the statistical analysis of categorical data.  Given 2 categorical random variables, X and Y, the chi-squared test of independence determines whether or not there exists a statistical dependence between them.  Formally, it is a hypothesis test with the following null and alternative hypotheses:

H_0: X \perp Y \ \ \ \ \ \text{vs.} \ \ \ \ \ H_a: X \not \perp Y

If you’re not familiar with probabilistic independence and how it manifests in categorical random variables, watch my video on calculating expected counts in contingency tables using joint and marginal probabilities.  For your convenience, here is another video that gives a gentler and more practical understanding of calculating expected counts using marginal proportions and marginal totals.

Today, I will continue from those 2 videos and illustrate how the chi-squared test of independence can be implemented in both R and SAS with the same example.

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Useful Options For Every SAS Program – Lessons and Resources from Dr. Jerry Brunner

Introduction

Today, I want to share some useful options to put at the beginning of every SAS program that I write.  These options will make the practicality of using SAS much easier.  My applied statistics professor from the University of Toronto, Jerry Brunner*, taught me some of these options when I first learned SAS in our class, and I’m grateful for that.  In later instances of using SAS in team projects, I have met SAS programmers who were delightfully surprised by the existence of these options and desperately wished that they had learned them earlier.  I hope that they will help you with your SAS programming.  I have also learned some useful options by posting questions on the SAS Support Communities online forum.

 

Clearing Output

After running your SAS program many times to test and debug, you will have accumulated numerous pages of old and useless output and log.  Scrolling through and searching for the desired portion to read in either file can be tedious and difficult.  Thus, it’s really helpful to have the option of clearing all of the output and the log whenever you run your script.  I put the following commands on top of every one of my SAS scripts.

/* 
Useful Options For Every SAS Program 
- With Some Tips Learned From Dr. Jerry Brunner
by Eric Cai - The Chemical Statistician
*/

dm 'cle log; cle out;';
ods html closeods html;

dm 'odsresults; clear';
ods listing close;
ods listing;

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