This blog post describes how South African ID numbers can be verified mathematically in Survey123. South African ID numbers have the following format:

{YYMMDD}{G}{SSS}{C}{A}{Z}

YYMMDD : Date of birth.
G  : Gender. 0-4 Female; 5-9 Male.
SSS  : Sequence No. for DOB/G combination.
C  : Citizenship. 0 SA; 1 Other.
A  : Usually 8, or 9
Z  : Control digit

The most challenging part of verifying the ID number is the control digit which is calculated by using the Luhn algorithm – this will be the focus of this blog post.

The best way to tackle complex mathematical functions in Survey123 is to break it up into separate mathematical calculations and using calculated fields:

The check digit is the last digit of the SA ID number so it can be retrieved with the following function: substr(\${idnr}, string-length(\${idnr}) – 1, string-length(\${idnr})) where \${idnr} refers to the captured ID number.

Once you have an understanding of the substr() function the rest of the calculations used to verify the ID number is pretty much straight forward.

The survey’s XLSForm file can be found here (and can be freely used): SA Id number