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. The data row is 1, 0, 1, 1. Choose the parity bit and correct reason for making the total number of 1s even.

    • Add 1: three 1s become four.
    • Add 0: four data bits is an even count.
    • Add no bit: even parity counts zeros, not ones.

  2. Why can an error-detection system send check bits with data?

    • To restore all changed data
    • To detect some data changes
    • To keep all messages secret

  3. A grid's row and column checks initially pass. Exactly one data bit flips and all check bits stay unchanged. The failed row and column cross at that bit.

    Circle one:   True   False

  4. A classroom code has data digits 2, 4, 1 and check digit 5. The total, including the check digit, must be a multiple of 10. What does the check say?

    • It passes: 5 is odd.
    • It passes: any check digit above 3 makes the total valid.
    • It fails: 12 is not a multiple of 10.

  5. All even-parity checks initially passed. Exactly one data bit flipped; check bits stayed unchanged. Find it. Check bits are in the last row and column.

    RowC1C2C3Check
    11100
    20010
    Check101-
    • Row 1, column 2
    • Row 2, column 3
    • Row 2, column 2

  6. Only one row's even parity is checked. Two data bits flip, but the row still passes. What does this show?

    • A pass proves no data changed.
    • Two flips can preserve parity.
    • A parity bit fixes both flips.

  7. The data digits are 3, 1 and 2. Choose one check digit from 0 to 9 so the total, including that digit, is a multiple of 10. Type the check digit.

    Answer: ______________

  8. All row and column checks initially pass. Check bits stay unchanged. Four data bits flip: (row 1, column 2), (row 1, column 4), (row 3, column 2), (row 3, column 4). Which checks fail?

    • The two row checks fail.
    • All checks keep passing.
    • Only column checks fail.
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Answer key

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

Extra bits that catch mistakes W1-mt_wKwax55eo--s1

  1. Add 1: three 1s become four. · The data contains three 1s. Add parity bit 1 to make four, an even count.
  2. To detect some data changes · Check bits let a receiver test a rule based on the data. They detect some errors, not every possible change.
  3. True · The single flip changes the parity of its own row and column, so their crossing locates it.
  4. It fails: 12 is not a multiple of 10. · 2 + 4 + 1 + 5 = 12. That is not a multiple of 10, so this code fails the stated check.
  5. Row 2, column 2 · Row 2 has an odd total of one 1, and column 2 also has one 1. The other checks pass. Their crossing is row 2, column 2.
  6. Two flips can preserve parity. · Two flips preserve even parity, so this single-row check can pass despite changed data.
  7. 4 · The data sum is 6. Adding 4 makes 10, a multiple of 10.
  8. All checks keep passing. · Each affected row has two flips, and each affected column has two. Every parity check still passes despite the four changes.
Worksheet · LightMySky