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Nearest neighbor matchingPowerPoint Presentation

Nearest neighbor matching

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### Nearest neighbor matching

### %propen(libname, dsname, idvariable, dependent, propensity)

### %propen(study,allpropen,id,athome,prob);

### /* Output control to one data set, intervention to another *//* Create random number to sort within group */

### /* Runs through control and experimental and matches */up to 20 subjects with identical propensity score */

USING THE GREEDY MATCH MACRO

Note: Much of the code originally was written by Lori Parsons

http://www2.sas.com/proceedings/sugi26/p214-26.pdf

This code has been written with simplicity as a primary concern.

If you do not have a large number of controls, you may want to modify it

/* Define the library for formats */

LIBNAME saslib "G:\oldpeople\sasdata\" ;

OPTIONS NOFMTERR FMTSEARCH = (saslib) ;

/* Define the library for study data */

LIBNAME study "C:\Users\AnnMaria\Documents\shrug\" ;

LIBNAME = directory for data sets

DSNAME = dataset with study data

IDVARIABLE = subject ID variable

DEPENDENT = dependent variable

PROPENSITY = propensity score produced in logistic regression

FOR EXAMPLE

Remember, we already have the study.allpropen dataset with the propensity score (prob) from the PROC LOGISTIC we just did

Propensity scores rounded to 5, then 4, 2, 3 and 1 decimals

%Do countr = 1 %to 5;

%let digits = %eval(6 - &countr);

%let roundto = %eval(10**&digits);

%let roundto = %sysevalf(1/&roundto);

%let nextin = %eval(&digits - 1);

MACRO NOTES

%Do countr = 1 %to 5;/* Starts %DO loop */

Use %EVAL function to do integer arithmetic

%let digits = %eval(6 - &countr);

Use %SYSEVALF function to do non-integers

Create 2 data sets */

DATA yes1 (KEEP= &probid_ydepend_yrandnum)

no1 (KEEP = &probid_ndepend_nrandnum);

SET in&digits;

We go through this loop 5 times and create data sets of records

matching to 5, 4, 3, 2 and 1 decimal places

We only keep four variables

Assignment statements */

randnum = RANUNI(0);

&prob = ROUND(&prob,&roundto);

Create a random number and

Round propensity score to a set number of digits

Output to Case Data set … */

IF &depend = 1 THEN DO ;

id_y = &id ;

depend_y = &depend ;

OUTPUT yes1 ;

END ;

We need to rename the dependent & id variables or they’ll get overwritten

… Or output control data set */

ELSE IF &depend = 0 THEN DO ;

id_n = &id ;

depend_n = &depend ;

OUTPUT no1 ;

END ;

Notice the data sets were named no1 and yes1

It becomes evident why shortly

%Do */i = 1 %to 20;

%let j = %eval(&i +1);

procsortdata = yes&i;

by &probrandnum;

datayes&iyes&j;

set yes&i;

by &prob;

if first.&prob then output yes&i;

else output yes&j;

NOTE: Matching without replacement

Same thing for controls */

procsortdata = no&i;

by &probrandnum;

datano&ino&j;

set no&i;

by &prob;

if first.&prob then output no&i;

else output no&j;

The randnum insures matching scores are pulled at random

Merge matches, end loop */

DATAmatch&i;

MERGE yes&i(in= ina)no&i(in= inb);

BY &prob;

IF ina AND inb;

run;

%END ;

/* Adds all matches into a single data set */ */

DATAallmatches;

SET

%DO k = 1 %TO 20;

match&k

%END ;

Concatenate all data sets with matches (N=20)

Create two data sets with IDs */

DATA

allyes (RENAME = (id_y = &id depend_y = &depend))

allno (RENAME = (id_n = &id depend_n = &depend));

SET allmatches ;

Create one file of all matched IDs */

DATAmatchfile;

SET allyesallno;

And sort it …

procsortdata = matchfile;

by &id &depend ;

proc sort data = */in&digits ; by &id &depend ;

/* Creates a data set of all subjects with */n-digit match */

/* Creates a second data set of subjects with no match */

data matches&digitsin&nextin ; merge in&digits (in = ina) matchfile (in= inb) ; by &id &depend ; if ina and inb then output matches&digits ; else output in&nextin ;

JUST A GOOD HABIT TO CHECK AS THE LOOP RUNS THROUGH */

Title "Matches &roundto " ;proc freq data = matches&digits ; tables &depend ;run ;%end ;End loop. Now match to 4 decimal places, etc

/* Adds 1- to 5-digit matches into a single data set */ */

data &lib..finalset;

set

%do m = 1 %to 5;

matches&m

%end ;

One final check & done ! */

Title "Distribution of Dependent Variable in &lib..finalset " ;

procfreqdata = &lib..finalset;

tables &depend ;

run;

%mendpropen;

run;

Did it work? */

** P <.01 **** P < .0001

Odds ratio */

How near? */

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