Side-by-side: Which Method Does What — Error Detection | IGCSE Computer Science

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 5% of your exam marks.

stable

Rare

Stable5%

Parity bits, checksums and check digits each appear in most papers. Often 3 to 4 marks.

Method	Detects	Locates	Corrects	Best for
Parity check	Single-bit errors (and other odd numbers of errors)	No	No	Fast, cheap link-level checks
Parity block	Single-bit errors	Yes (intersection of row + column)	Single-bit only	Memory chips, simple data blocks
Checksum	Most random corruption	No	No	Files, packets, web downloads
Echo check	Almost any corruption	No (and ambiguous about direction)	No (use ARQ on top)	Short links where reliability matters more than speed
Check digit	Single wrong digit, transpositions, omissions	No	No	Human-entered numerical codes (ISBN, barcodes, bank cards)
ARQ	Uses another method (e.g. checksum)	Depends	Yes (via re-transmission)	Combined with detection for reliable delivery

Choosing the right tool

A typical exam question gives a scenario and asks which method best fits. The decision tree:

Is the data human-entered numbers? → Check digit.
Is the data a transmitted byte or packet? → Parity (very fast, weak) or checksum (slower, stronger).
Does the system need to know exactly which bit is wrong? → Parity block.
Does the system need to automatically recover from errors, not just detect them? → ARQ on top of any of the above.

The next topic in this chapter, Encryption (topic 6), addresses a different concern: not whether data is correct, but whether it can be read by anyone who intercepts it.