SAS Learning Module
Creating and recoding variables in SAS

1. Creating and replacing variables in SAS

We will illustrate creating and replacing variables in SAS using a data file about 26 automobiles with their make, price, mpg, repair record in 1978 (rep78), and whether the car was foreign or domestic (foreign).  The program below reads the data and creates a temporary data file called "auto".  Please note that there are two missing values for mpg in the data file (coded as a single period). 

We will create one new variable to go along with the existing ones. First, we will create cost so that it gives us the price in thousands of dollars. Then we will create mpgpd which will stand for miles per gallon per thousand dollars.  In each case, we just type the variable name, followed by an equal sign, followed by an expression for the value.

DATA auto;
  INPUT make $ price mpg rep78 foreign;
  cost = ROUND( price / 1000 );
  mpgptd = mpg / price;
DATALINES;
AMC    4099 22 3 0
AMC    4749 17 3 0
AMC    3799 22 3 0
Audi   9690  . 5 1
Audi   6295 23 3 1
BMW    9735 25 4 1
Buick  4816 20 3 0
Buick  7827 15 4 0
Buick  5788 18 3 0
Buick  4453 26 3 0
Buick  5189 20 3 0
Buick 10372 16 3 0
Buick  4082 19 3 0
Cad.  11385 14 3 0
Cad.  14500 14 2 0
Cad.  15906 21 3 0
Chev.  3299 29 3 0
Chev.  5705 16 4 0
Chev.  4504  . 3 0
Chev.  5104 22 2 0
Chev.  3667 24 2 0
Chev.  3955 19 3 0
Datsun 6229 23 4 1
Datsun 4589 35 5 1
Datsun 5079 24 4 1
Datsun 8129 21 4 1
;
RUN;
PROC PRINT DATA=auto;
RUN;

Here is the output of the proc print.  You can compare the output to the original data.

OBS    MAKE      PRICE    MPG    REP78    FOREIGN    COST     MPGPTD

  1    AMC        4099     22      3         0         4     .0053672
  2    AMC        4749     17      3         0         5     .0035797
  3    AMC        3799     22      3         0         4     .0057910
  4    Audi       9690      .      5         1        10     .       
  5    Audi       6295     23      3         1         6     .0036537
  6    BMW        9735     25      4         1        10     .0025681
  7    Buick      4816     20      3         0         5     .0041528
  8    Buick      7827     15      4         0         8     .0019164
  9    Buick      5788     18      3         0         6     .0031099
 10    Buick      4453     26      3         0         4     .0058388
 11    Buick      5189     20      3         0         5     .0038543
 12    Buick     10372     16      3         0        10     .0015426
 13    Buick      4082     19      3         0         4     .0046546
 14    Cad.      11385     14      3         0        11     .0012297
 15    Cad.      14500     14      2         0        15     .0009655
 16    Cad.      15906     21      3         0        16     .0013203
 17    Chev.      3299     29      3         0         3     .0087905
 18    Chev.      5705     16      4         0         6     .0028046
 19    Chev.      4504      .      3         0         5     .       
 20    Chev.      5104     22      2         0         5     .0043103
 21    Chev.      3667     24      2         0         4     .0065449
 22    Chev.      3955     19      3         0         4     .0048040
 23    Datsun     6229     23      4         1         6     .0036924
 24    Datsun     4589     35      5         1         5     .0076269
 25    Datsun     5079     24      4         1         5     .0047253
 26    Datsun     8129     21      4         1         8     .0025833

Note that cost is just a one or two-digit value. The vehicle that achieves the best mpgptd is the Chev. for observation 17 which gets 9+ miles per gallon for every thousand dollars in price. The Cad. in observation 14 has the worst mpgptd.

Also note that there are two missing values for mpgptd because of the missing values in mpg.

2. Recoding variables in SAS

The variable rep78 is coded 1 through 5 standing for poor, fair, average, good and excellent. We would like to change rep78 so that it has only three values, 1 through 3, standing for below average, average, and above average. We will do this by creating a new variable called repair and recoding the values of rep78 into it.

We will also create a new variable called himpg that is a dummy coding of mpg. All vehicles with better than 20 mpg will be coded 1 and those with 20 or less will be coded 0.

SAS does not have a recode command, so we will use a series of if-then/else commands in a data step to do the job. This data step creates a temporary data file called auto2

DATA auto2;
  SET auto;
     	 
  repair = .;
  IF (rep78=1) or (rep78=2) THEN repair = 1;
  IF (rep78=3) THEN repair = 2;
  IF (rep78=4) or (rep78=5) THEN repair = 3;
   
  himpg = .;
  IF (mpg <= 20) THEN himpg = 0; 
  IF (mpg >  20) THEN himpg = 1;
RUN;

Note that we begin by setting repair and himpg to missing, just in case we make a mistake in the recoding.  Proc freq will show us how the recoding worked.

PROC FREQ DATA=auto2;
  TABLES repair*rep78 repair*himpg / MISSING;
RUN;	 
TABLE OF REPAIR BY REP78

REPAIR     REP78

Frequency|
Percent  |
Row Pct  |
Col Pct  |       2|       3|       4|       5|  Total
---------+--------+--------+--------+--------+
       1 |      3 |      0 |      0 |      0 |      3
         |  11.54 |   0.00 |   0.00 |   0.00 |  11.54
         | 100.00 |   0.00 |   0.00 |   0.00 |
         | 100.00 |   0.00 |   0.00 |   0.00 |
---------+--------+--------+--------+--------+
       2 |      0 |     15 |      0 |      0 |     15
         |   0.00 |  57.69 |   0.00 |   0.00 |  57.69
         |   0.00 | 100.00 |   0.00 |   0.00 |
         |   0.00 | 100.00 |   0.00 |   0.00 |
---------+--------+--------+--------+--------+
       3 |      0 |      0 |      6 |      2 |      8
         |   0.00 |   0.00 |  23.08 |   7.69 |  30.77
         |   0.00 |   0.00 |  75.00 |  25.00 |
         |   0.00 |   0.00 | 100.00 | 100.00 |
---------+--------+--------+--------+--------+
Total           3       15        6        2       26
            11.54    57.69    23.08     7.69   100.00


TABLE OF REPAIR BY HIMPG

REPAIR     HIMPG

Frequency|
Percent  |
Row Pct  |
Col Pct  |       0|       1|  Total
---------+--------+--------+
       1 |      1 |      2 |      3
         |   3.85 |   7.69 |  11.54
         |  33.33 |  66.67 |
         |   7.69 |  15.38 |
---------+--------+--------+
       2 |      9 |      6 |     15
         |  34.62 |  23.08 |  57.69
         |  60.00 |  40.00 |
         |  69.23 |  46.15 |
---------+--------+--------+
       3 |      3 |      5 |      8
         |  11.54 |  19.23 |  30.77
         |  37.50 |  62.50 |
         |  23.08 |  38.46 |
---------+--------+--------+
Total          13       13       26
            50.00    50.00   100.00

Uh oh, there's a problem with himpg. There are no missing values for himpg even though there were two missing values of mpg.  SAS treats missing values (values coded with a . ) as the smallest number possible (i.e., negative infinity).  When we recoded mpg we wrote

IF (mpg <= 20) THEN himpg = 0; 

which converted all values of mpg that were 20 or less into a value of 0 for himpg.  Since a missing value is also less than 20, the missing values got recoded to 0 as well.  (It is unforeseen mistakes like this that make it so important to check every variable that you recode.)  Let's try recoding himpg again, being careful to properly treat missing values like this:

IF (. < mpg <= 20) THEN himpg = 0; 

The complete program, with the fixed if statement, is shown below.

DATA auto2;
  SET auto;
     	 
  repair = .;
  IF (rep78=1) or (rep78=2) THEN repair = 1;
  IF (rep78=3) THEN repair = 2;
  IF (rep78=4) or (rep78=5) THEN repair = 3;
   
  himpg = .;
  IF (.  < mpg <= 20) THEN himpg = 0; 
  IF (mpg >  20) THEN himpg = 1;
RUN;

Now let's use proc freq again to check the recoding.

PROC FREQ DATA=auto2;
  TABLES repair*himpg / MISSING;
RUN;
TABLE OF REPAIR BY HIMPG

REPAIR     HIMPG

Frequency|
Percent  |
Row Pct  |
Col Pct  |       .|       0|       1|  Total
---------+--------+--------+--------+
       1 |      0 |      1 |      2 |      3
         |   0.00 |   3.85 |   7.69 |  11.54
         |   0.00 |  33.33 |  66.67 |
         |   0.00 |   9.09 |  15.38 |
---------+--------+--------+--------+
       2 |      1 |      8 |      6 |     15
         |   3.85 |  30.77 |  23.08 |  57.69
         |   6.67 |  53.33 |  40.00 |
         |  50.00 |  72.73 |  46.15 |
---------+--------+--------+--------+
       3 |      1 |      2 |      5 |      8
         |   3.85 |   7.69 |  19.23 |  30.77
         |  12.50 |  25.00 |  62.50 |
         |  50.00 |  18.18 |  38.46 |
---------+--------+--------+--------+
Total           2       11       13       26
             7.69    42.31    50.00   100.00

There, that's better, this time there are two missing values for himpg.

3. Problems to look out for
 

Watch out for math errors, such as, division by zero and square root of a negative number.

4. Helpful hints and suggestions

  • Set values to missing and then recode them.
  • Use new variable names when you create or recode variables. Avoid constructions like this, total = total + sub1 + sub2; that reuse the variable name total.
  • Use the missing option with proc freq to make sure all missing values are accounted for.

5. For more information

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