Parity Check

A parity check is the simplest error detection method in data transmission. A single check bit (parity bit) is appended to a data sequence so that the total number of 1-bits is always even (even parity) or odd (odd parity). The receiver counts the 1-bits; a mismatch signals an error.

In QR codes, parity concepts protect format information (15 bits encoding error correction level and mask pattern) and version information (18 bits). These fields use BCH codes - a generalization of parity checking - and are recorded twice near the finder patterns for redundancy.

A simple parity check detects only single-bit errors and cannot correct them. QR code data payloads use Reed-Solomon codes, which can recover up to 30% data loss. Parity checking is the foundational principle: adding redundant bits to verify integrity underlies checksums, CRCs, and all error detection techniques.