SAS Tips: Verifying the check digit in Swedish national ID numbers

Last updated on Mar 9, 2019

Swedish personal identification numbers (PNRs) comprise 10 digits (12 if century of birth is included). The first 6 digits represent date of birth (YYMMDD), the ninth digit represents gender (even for females and odd for males), and the tenth digit is a check digit which can be constructed from the preceding nine digits. SAS code for verifying PNRs is shown below. This code can be downloaded here.

Algorithm for calculating the check digit

Consider the PNR 310317-099x. First multiply each of the first nine digits in the PNR by the digits 2,1,2,1,2,1,2,1,2 and calculate the cumulative sum of each of these 9 calculations. If the product results in a number greater than 10, then add the individual digits. For example, 9 times 2 = 18, so we add 9 (the sum of 1 and 8) to the cumulative sum in the example below.

3	1	0	3	1	7	0	9	9
2	1	2	1	2	1	2	1	2
6	1	0	3	2	7	0	9	18	37

In the example above, the cumulative sum is 37. The check digit is the number we would have to add to the product sum in order to obtain a multiple of 10. In the above example, we would need to add 3, so the check digit is 3. If the cumulative sum is a multiple of 10 then the check digit is 0.

SAS code for verifying the check digit

/*******************************************
PNR_CHK.SAS
This code reads 10-digit person numbers and
checks that the check digit (the 10th digit
in the PNR) is correct.
It is assumed that PNR is a character
variable of length 10.

Paul Dickman (paul.dickman@ki.se)
September 1999
*******************************************/

/*******************
Read some test data
********************/
data temp;
input pnr $ 1-10;
cards;
3103170993
3103170999
6812241450
6812241457
;
run;

data pnr_chk;
set temp;
length product $ 18 result $ 3;
array two_one {9} (2 1 2 1 2 1 2 1 2);
/*********************************************
multiply each of the first 9 digits in PNR by
the corresponding digit in the array two_one
and concatenate the result.
The COMPRESS function removes blanks.
*********************************************/
do i = 1 to 9;
product=compress(product||(substr(pnr,i,1)
                         *two_one{i}));
end;

/** Now we sum the digits **/
do i = 1 to length(product);
sum=sum(sum,substr(product,i,1));
end;

/** extract the check digit from PNR **/
chk=substr(pnr,10,1);

/** calculate the correct check digit **/
corr_chk=mod(10-mod(sum,10),10);

if chk=corr_chk then result='ok';
else result='bad';
label
chk='Actual check number'
corr_chk='Correct check number'
pnr='Personnummer (10 digits)'
;
run;

proc print data=pnr_chk;
var pnr chk product sum corr_chk result;
run;