lesson 4 - topics creating new variables in the data step sas functions

Lesson 4 - Topics

• Creating new variables in the data step

• SAS Functions

Creating New Variables

• Direct assignments(formulas):c = a + b ;d = 2*a + 3*b + 7*c ;bmi = weight/(height*height);

• Indirect assignments (if/then/else)if age < 50 then young = 1; else young = 2;

if income < 15 then tax = 1; elseif income < 25 then tax = 2; elseif income >=25 then tax = 3;

Direct Assignments(Formulas)

• Examplec = a + b ;

So if a = 2, b =3, c = 5;

What if a is missing, what is c? C will be missing

What if b is missing?

If/then/else Statements

With if-then-else definitions SAS stops executing after the first true statement

if income < 15 then tax = 1; elseif income < 25 then tax = 2; elseif income >=25 then tax = 3;

What if income is 10?What if income is 23?What if income is 30?What if income is missing?

Tax = 1Tax = 2Tax = 3Tax = 1

Create a new variable with 2 levels, one for college graduates and one for non-college graduates.

Creating New Variables

Program 5DATA tdata;INFILE ‘C:\SAS_Files\tomhs.data' ;INPUT @ 1 ptid $10. @ 49 educ 1. @123 sbp12 3. ;

* This way will code missing values to the value 2;if educ < 7 then grad1 = 2 ; elseif educ >=7 then grad1 = 1 ;

* The next two ways are equivalent and are correct;if educ < 7 and educ ne . then grad2 = 2; elseif educ >=7 then grad2 = 1;

* IN is a useful function in SAS ;if educ IN(1,2,3,4,5,6) then grad3 = 2; elseif educ IN(7,8,9) then grad3 = 1;

New variable defines go after the input statement

PROC FREQ DATA=tdata; TABLES educ grad1 grad2 grad3 ; Cumulative Cumulativeeduc Frequency Percent Frequency Percent--------------------------------------------------------- 1 3 3.03 3 3.03 3 4 4.04 7 7.07 4 23 23.23 30 30.30 5 14 14.14 44 44.44 6 12 12.12 56 56.57 7 16 16.16 72 72.73 8 10 10.10 82 82.83 9 17 17.17 99 100.00

Frequency Missing = 1

Cumulative Cumulativegrad1 Frequency Percent Frequency Percent----------------------------------------------------------- 1 43 43.00 43 43.00 2 57 57.00 100 100.00





Coded the missing value for educ to 2

PROC FREQ DATA=tdata; TABLES educ*grad1 /MISSING NOCUM NOPERCENT NOROW NOCOL; TITLE 'Use Crosstabulation to Verify Recoding'; RUN;

Table of educ by grad1

educ grad1

Frequency‚ 1‚ 2‚ Totalƒƒƒƒƒƒƒƒƒˆƒƒƒƒƒƒƒƒˆƒƒƒƒƒƒƒƒˆ . ‚ 0 ‚ 1 ‚ 1ƒƒƒƒƒƒƒƒƒˆƒƒƒƒƒƒƒƒˆƒƒƒƒƒƒƒƒˆ 1 ‚ 0 ‚ 3 ‚ 3ƒƒƒƒƒƒƒƒƒˆƒƒƒƒƒƒƒƒˆƒƒƒƒƒƒƒƒˆ 3 ‚ 0 ‚ 4 ‚ 4ƒƒƒƒƒƒƒƒƒˆƒƒƒƒƒƒƒƒˆƒƒƒƒƒƒƒƒˆ 4 ‚ 0 ‚ 23 ‚ 23ƒƒƒƒƒƒƒƒƒˆƒƒƒƒƒƒƒƒˆƒƒƒƒƒƒƒƒˆ 5 ‚ 0 ‚ 14 ‚ 14ƒƒƒƒƒƒƒƒƒˆƒƒƒƒƒƒƒƒˆƒƒƒƒƒƒƒƒˆ 6 ‚ 0 ‚ 12 ‚ 12ƒƒƒƒƒƒƒƒƒˆƒƒƒƒƒƒƒƒˆƒƒƒƒƒƒƒƒˆ 7 ‚ 16 ‚ 0 ‚ 16ƒƒƒƒƒƒƒƒƒˆƒƒƒƒƒƒƒƒˆƒƒƒƒƒƒƒƒˆ 8 ‚ 10 ‚ 0 ‚ 10ƒƒƒƒƒƒƒƒƒˆƒƒƒƒƒƒƒƒˆƒƒƒƒƒƒƒƒˆ 9 ‚ 17 ‚ 0 ‚ 17ƒƒƒƒƒƒƒƒƒˆƒƒƒƒƒƒƒƒˆƒƒƒƒƒƒƒƒˆTotal 43 57 100

This shows that the missing value for educ got assigned a value of 2

* Recode sbp12 into 3 levels;

if sbp12 = . then sbp12c = . ; elseif sbp12 < 120 then sbp12c = 1 ; elseif sbp12 < 140 then sbp12c = 2 ; elseif sbp12 >=140 then sbp12c = 3 ; With if-then-else definitions SAS stops executing after the first true statement

Values < 120 will be assigned value of 1

Values 120-139 will be assigned value of 2

Values >=140 will be assigned value of 3

Missing values will be assigned to missing

PROC FREQ DATA=tdata; TABLES sbp12c sbp12;RUN;

OUTPUT Cumulative Cumulativesbp12c Frequency Percent Frequency Percentƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒ 1 36 39.13 36 39.13 2 43 46.74 79 85.87 3 13 14.13 92 100.00


Cumulative Cumulativesbp12 Frequency Percent Frequency Percentƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒ 93 1 1.09 1 1.09 94 1 1.09 2 2.17 101 1 1.09 3 3.26 104 1 1.09 4 4.35 105 1 1.09 5 5.43 (more values) 147 1 1.09 87 94.57 148 1 1.09 88 95.65 149 1 1.09 89 96.74 153 1 1.09 90 97.83 154 1 1.09 91 98.91 158 1 1.09 92 100.00


* Easy but costly error to make;

if sbp12 = . then sbp12c = . ; elseif sbp12 < 120 then sbp12c = 1 ; elseif sbp12 < 140 then sbp12 = 2 ; elseif sbp12 >=140 then sbp12c = 3 ;

PROC FREQ DATA=tdata; TABLES sbp12c;RUN;

The FREQ Procedure

Cumulative Cumulativesbp12c Frequency Percent Frequency Percentƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒ 1 36 73.47 36 73.47 3 13 26.53 49 100.00


How come no values of 2 and why so many missing?

Important Facts When Creating New Variable

1. New variables are initialized to missing

2. Missing values are < any value

if var < value (true if var is missing)

3. Reference missing values for numeric variables as .

4. Reference missing values for character variables as ' '

if sbp = . then ... (or if missing(sbp))

if clinic = ' ' then ...

SAS Handling of Missing Data When Creating New Variables

• Direct assignments(formulas):c = a + b ;d = 2*a + 3*b + 7*c ;bmi = weight/(height*height);

If any variable on the right-hand side is missing then the new variable will be missing

• Indirect assignmentsif age < 50 then young = 1; else young=2;

New variables are initialized to missing but may be given a value if any of the IF statements are true

What Value to Set New Variable

if age < 20 then teenager = 1; elseif age >=20 then teenager = 2;

if age < 20 then teenager = 1; elseif age >=20 then teenager = 0;

if age < 20 then teenager = ‘YES’; elseif age >=20 then teenager = ‘NO’;

* Program 6 SAS Functions ;

DATA example;INFILE ‘C:\SAS_Files\tomhs.data' ;INPUT @058 height 4.1 @085 weight 5.1 @172 ursod 3. @236 (se1-se10) (1.0 + 1); bmi = (weight*703.0768)/(height*height);rbmi1 = ROUND(bmi,1);lursod = LOG(ursod);

seavg = MEAN (OF se1-se10); semax = MAX (OF se1-se10);semin = MIN (OF se1-se10);

* Use of dash notation ;

seavg = MEAN (OF se1-se10);

This is the same as

seavg = MEAN (se1,se2,se3,se4,se5,se6,se7,se8,se9,se10);

The OF is very important. Otherwise SAS thinks you are subtracting se10 from se1.

To use this notation the ROOT of the name must be the same.

* Two ways of computing average ;

seavg = MEAN (se1,se2,se3,se4,se5,se6,se7,se8,se9,se10);

Versus

seavg = (se1+se2+se3+se4+se5+se6+se7+se8+se9+se10)/10;

Using mean function computes the average of non-missing values. Result is missing only if all values all missing.

Using + formula requires all values be non-missing otherwise result will be missing

if N(of se1-se10) > 5 then seavg = MEAN(of se1-se10);

What does this statement do?

PROC PRINT DATA = example (OBS=15); VAR bmi rbmi1 rbmi2 seavg semin semax ; TITLE 'Listing of Selected Data for 15 Patients ';RUN;

PROC FREQ DATA = example; TABLES semax; TITLE 'Distribution of Worse Side Effect Value'; TITLE2 'Side Effect Scores Range from 1 to 4';RUN;

ods graphics on;PROC UNIVARIATE DATA = example ; VAR ursod lursod; QQPLOT ursod lursod; TITLE 'Quantile Plots for Urine Sodium Data';RUN;

Listing of Selected Data for 10 Patients

Obs bmi rbmi1 seavg semin semax

1 28.2620 28 1.1 1 2

2 35.9963 36 1.0 1 1

3 27.0489 27 1.0 1 1

4 28.2620 28 1.1 1 2

5 33.2008 33 1.0 1 1

6 27.7691 28 1.2 1 2

7 32.6040 33 1.0 1 1

8 22.4057 22 1.2 1 2

9 37.2037 37 1.1 1 2

10 33.1717 33 1.7 1 3

Distribution of Worse Side Effect ValueSide Effect Scores Ranges from 1 to 4

The FREQ Procedure

Cumulative Cumulative

semax Frequency Percent Frequency Percent

----------------------------------------------------------

1 33 33.00 33 33.00

2 52 52.00 85 85.00

3 13 13.00 98 98.00

4 2 2.00 100 100.00

2 patients had at least 1 severe side effect

Log transformed value shows a better linear pattern

lesson 4 - topics creating new variables in the data step sas functions

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