Spotting a Mistake in Stored or Sent Data · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Extra bits that catch mistakes

Computing · Computing Fundamentals · ages 13-14
Name ______________________   Date ____________

  1. A row must hold an even count of ones. It currently holds 1, 0, 1, 1. What is its parity bit?

    • 1, since three ones need one more for even
    • 0, since the row is already even
    • 1, since parity bits are always 1

  2. Why do systems send extra bits along with data?

    • To make the message longer
    • To catch errors without reading the message
    • To hide the message from readers

  3. In a parity grid, the broken row and column cross at the flipped bit.

    Circle one:   True   False

  4. A code shows digits 2, 4, 1 with check digit 5 under the add-to-10 rule. What does the check say?

    • It passes, since 5 is odd
    • It passes, since checks are decoration
    • It fails, since 2, 4, 1 need 3 to reach 10

  5. Row 2 and column 3 fail parity in a grid. Where is the flipped bit?

    • Row 3, column 2
    • Row 1, column 1
    • Row 2, column 3

  6. Two flipped bits cancel and pass parity. What does this teach?

    • Detection proves data is fully clean
    • Detection catches change but cannot promise all is well
    • Parity bits cause the errors they find

  7. A code shows digits 3, 1 and 2 under the same add-to-10 rule. Type its check digit.

    Answer: ______________

  8. Mara's grid shows broken parity in two rows and one column. How many flipped bits at least?

    • One flipped bit explains everything
    • At least two flipped bits
    • No flipped bits at all
LightMySky · lightmysky.comW1-mt_wKwax55eo--s1

Answer key

For grown-ups. Fold this page away before handing over the rest.

Extra bits that catch mistakes W1-mt_wKwax55eo--s1

  1. 1, since three ones need one more for even · Three ones are odd, so the parity bit is 1 to reach even.
  2. To catch errors without reading the message · Extra bits fit only when nothing changed.
  3. True · Only the flipped bit sits in both broken lines.
  4. It fails, since 2, 4, 1 need 3 to reach 10 · One wrong digit breaks the total, which fails the check.
  5. Row 2, column 3 · The crossing of the two broken lines holds the error.
  6. Detection catches change but cannot promise all is well · Detection reports a broken fit, never a clean bill of health.
  7. 4 · They add to 6, and 4 brings the total to 10.
  8. At least two flipped bits · One flip breaks exactly one row, so two broken rows need two flips.
Worksheet · LightMySky