tarkistusluvut

Tarkistusluvut are digits appended to a number, forming a sequence that makes it possible to detect certain errors in transcription or entry. They function as a simple form of error-detecting code and are widely used in identification numbers, account codes, and barcodes. The basic idea is that the check digit is computed from the other digits with a mathematical rule, so recalculating the rule on arrival should yield the same check digit if the number is correct.

Common tarkistusluvut and their algorithms include several well-known methods. The Luhn algorithm, widely used for credit and debit card numbers, works by doubling every second digit, subtracting 9 if the result is greater than 9, summing all digits, and choosing a final digit that makes the total a multiple of 10. ISBN-13 uses a modulo-10 check with alternating weights of 1 and 3 for the first 12 digits, and the check digit is the amount needed to reach a multiple of 10. ISBN-10 uses a modulo-11 scheme with the final digit possibly being X to represent 10. ISSN uses a modulo-11 check with the digit 10 represented by X. More robust systems include Verhoeff and Damm algorithms, designed to detect a wider range of common errors.

Tarkistusluvut are common in banking, publishing, and library systems, where they help catch accidental errors during data entry or transmission. They do not prevent all mistakes but substantially reduce the likelihood of accepting incorrect numbers.