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Error Checking: Guide

Identifying discrepancies between data sets

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Error Types

Common types of data entry errors

  1. Transposition Errors
  2. Substitution Errors
  3. Omission Errors
  4. Addition Errors
  5. Double Transposition
  6. Visual Confusion
  7. Keyboard Proximity
  8. Repeated Character
  9. Reversal
  10. Spacing Error
  11. Format Shift
  12. Phonetic Substitution
  13. Case Flip
  1. Rule Validation

Learn 1

Transposition Errors

Adjacent characters are swapped in position.

Transposition errors occur when two adjacent characters swap positions. This is one of the most common data entry errors, often caused by typing too quickly or by dyslexia. Examples include typing '1324' instead of '1234' or 'hte' instead of 'the'. These errors are subtle because all the correct characters are present.

Original: ABCD → Error: ABDC (C and D swapped)

  • 12345 → 12435 (3 and 4 transposed)
  • ABCDEF → ABDCEF (C and D transposed)
  • AB-1234 → AB-1324 (2 and 3 transposed)

How to recognize it

  • Same characters, but two adjacent ones are in reversed order
  • Length remains the same
  • Often occurs in the middle of strings

Common mistakes

  • Confusing transposition with substitution
  • Missing subtle swaps in long strings
  • Not checking adjacent pairs systematically

Step-by-step walkthrough

Find the error: Original '12345' vs Given '12435'

  1. Compare position 1: '1' = '1' ✓
  2. Compare position 2: '2' = '2' ✓
  3. Compare position 3: '3' vs '4' - mismatch found!
  4. Compare position 4: '4' vs '3' - mismatch found!
  5. Compare position 5: '5' = '5' ✓
  6. Pattern: positions 3 and 4 contain swapped characters

Answer: Transposition error: '3' and '4' swapped at positions 3-4

Learn 2

Substitution Errors

One character is replaced with a different character.

Substitution errors occur when one character is incorrectly replaced with another. This often happens with visually similar characters (0 vs O, 1 vs l, 5 vs S) or adjacent keyboard keys. The string length stays the same, but one character is wrong. These errors are common in manual data entry.

Original: ABCD → Error: ABXD (C replaced with X)

  • 12345 → 12845 (3 replaced with 8)
  • JOHNSON → JOHNS0N (O replaced with zero)
  • AB-1234 → AB-1284 (3 replaced with 8)

How to recognize it

  • One character is different, all others match
  • Length remains the same
  • Often involves similar-looking characters

Common mistakes

  • Confusing substitution with transposition
  • Missing errors with visually similar characters (0/O, 1/l/I)
  • Not comparing character by character

Step-by-step walkthrough

Find the error: Original 'JOHNSON' vs Given 'JOHNS0N'

  1. Compare position 1: 'J' = 'J'
  2. Compare position 2: 'O' = 'O'
  3. Compare position 3: 'H' = 'H'
  4. Compare position 4: 'N' = 'N'
  5. Compare position 5: 'S' = 'S'
  6. Compare position 6: 'O' vs '0' - mismatch found
  7. Compare position 7: 'N' = 'N'
  8. The letter 'O' was replaced with the digit '0' (zero)

Answer: Substitution error: letter 'O' replaced with digit '0' at position 6

Learn 3

Omission Errors

One or more characters are missing from the data.

Omission errors occur when characters are accidentally left out during data entry. This can happen from skipping a keystroke, misreading source data, or copying errors. The result is a shorter string than the original. These errors can be caught by checking string length first.

Original: ABCD → Error: ABD (C omitted)

  • 123456 → 12346 (5 omitted)
  • WILLIAMS → WILLAMS (I omitted)
  • AB-1234 → AB-124 (3 omitted)

How to recognize it

  • Copy is shorter than original
  • All present characters match, one is missing
  • Check length first as quick filter

Common mistakes

  • Not noticing the length difference
  • Confusing omission with substitution of similar characters
  • Missing repeated character omissions (e.g., 'MISS' vs 'MIS')

Step-by-step walkthrough

Find the error: Original '123456' vs Given '12346'

  1. Check lengths: original has 6 characters, given has 5
  2. Compare position 1: '1' = '1'
  3. Compare position 2: '2' = '2'
  4. Compare position 3: '3' = '3'
  5. Compare position 4: '4' = '4'
  6. Compare position 5: '5' vs '6' - mismatch, '5' is missing
  7. The digit '5' was omitted from position 5

Answer: Omission error: character '5' is missing from position 5

Learn 4

Addition Errors

Extra characters are accidentally inserted into the data.

Addition errors occur when extra characters are accidentally inserted during data entry. This can happen from double-tapping keys, misreading source data, or accidentally hitting adjacent keys. The result is a longer string than the original. Like omission errors, these can be quickly detected by checking length.

Original: ABCD → Error: ABCXD (X inserted)

  • 12345 → 123345 (extra 3 inserted)
  • SMITH → SMIITH (extra I inserted)
  • AB-1234 → AB-12334 (extra 3 inserted)

How to recognize it

  • Copy is longer than original
  • Contains all original characters plus extras
  • Often a repeated or adjacent character

Common mistakes

  • Not noticing the length difference
  • Missing subtle additions in long strings
  • Confusing with doubled legitimate characters

Step-by-step walkthrough

Find the error: Original '12345' vs Given '123345'

  1. Check lengths: original has 5 characters, given has 6
  2. Compare position 1: '1' = '1'
  3. Compare position 2: '2' = '2'
  4. Compare position 3: '3' = '3'
  5. Compare position 4: '4' vs '3' - extra character found
  6. An extra '3' was inserted at position 4

Answer: Addition error: extra '3' inserted at position 4

Learn 5

Double Transposition

Two separate pairs of adjacent characters are swapped.

Double transposition errors occur when two separate pairs of adjacent characters are swapped in the same string. This creates a more complex error pattern that can be harder to detect because the errors are distributed across the string.

Original: ABCDEF → Error: BADCEF (AB swapped AND CD swapped)

  • 123456 → 213465 (12 swapped AND 45 swapped)
  • ABCDEF → BACDEF → BADCEF (two transpositions)
  • AB-1234 → BA-1324 (AB swapped AND 23 swapped)

How to recognize it

  • Two separate pairs of adjacent characters are reversed
  • Length remains the same
  • All original characters are present

Common mistakes

  • Missing one of the two transpositions
  • Confusing with single transposition
  • Not scanning the entire string

Step-by-step walkthrough

Find the error: Original '123456' vs Given '213465'

  1. Compare position 1: '1' vs '2' - mismatch
  2. Compare position 2: '2' vs '1' - mismatch
  3. First swap identified: '1' and '2' are transposed
  4. Compare position 3: '3' = '3'
  5. Compare position 4: '4' vs '4'
  6. Compare position 5: '5' vs '6' - mismatch
  7. Compare position 6: '6' vs '5' - mismatch
  8. Second swap identified: '5' and '6' are transposed

Answer: Double transposition error: '1-2' swapped at positions 1-2 AND '5-6' swapped at positions 5-6

Learn 6

Visual Confusion

Similar-looking characters are mistaken for each other.

Visual confusion errors occur when characters that look similar are substituted for one another. Common confusions include B/8, O/0, I/1, S/5, G/6, and Z/2. These errors often occur when reading handwritten data or when working with low-quality displays.

Common pairs: B↔8, O↔0, I↔1, S↔5, G↔6, Z↔2

  • AB-1234 → A8-1234 (B confused with 8)
  • CODE01 → CODEO1 (0 confused with O)
  • SIGNAL → 5IGNAL (S confused with 5)

How to recognize it

  • Character replaced with visually similar alternative
  • Length remains the same
  • Often involves letter/number confusion

Common mistakes

  • Not recognizing visually similar pairs
  • Assuming digits are always correct
  • Missing context clues (letters vs numbers sections)

Step-by-step walkthrough

Find the error: Original 'AB-1234' vs Given 'A8-1234'

  1. Compare position 1: 'A' = 'A'
  2. Compare position 2: 'B' vs '8' - mismatch found
  3. Compare position 3: '-' = '-'
  4. Compare remaining: '1234' = '1234'
  5. The letter 'B' was confused with the digit '8'
  6. These characters look similar: B and 8 have similar shapes

Answer: Visual confusion error: 'B' mistaken for '8' at position 2 (similar appearance)

Learn 7

Keyboard Proximity

A character is replaced by an adjacent key on the keyboard.

Keyboard proximity errors occur when a typist accidentally presses a key adjacent to the intended key on a QWERTY keyboard. For example, pressing 'R' instead of 'E', or '5' instead of '4'. These are among the most common typing errors.

Key replaced by horizontally or vertically adjacent key on QWERTY layout

  • HELLO → HRLLO (E replaced by adjacent R)
  • 12345 → 12445 (3 replaced by adjacent 4)
  • CODE → CODW (E replaced by adjacent W)

How to recognize it

  • Single character replaced by keyboard neighbor
  • Length remains the same
  • Check QWERTY layout for adjacent keys

Common mistakes

  • Not visualizing the keyboard layout
  • Confusing with random substitution
  • Forgetting diagonal adjacencies

Step-by-step walkthrough

Find the error: Original 'HELLO' vs Given 'HRLLO'

  1. Compare position 1: 'H' = 'H'
  2. Compare position 2: 'E' vs 'R' - mismatch found
  3. Compare position 3: 'L' = 'L'
  4. Compare position 4: 'L' = 'L'
  5. Compare position 5: 'O' = 'O'
  6. Check keyboard layout: 'E' and 'R' are adjacent on QWERTY keyboard
  7. The typist hit 'R' instead of 'E' due to finger placement

Answer: Keyboard proximity error: 'E' replaced by adjacent key 'R' at position 2

Learn 8

Repeated Character

A character is accidentally typed twice in succession.

Repeated character errors occur when a typist accidentally presses a key twice, creating an unwanted duplicate character. This is common with fast typing or when a key is slightly sticky. The result is a string that is one character longer than intended.

Original character at position N is duplicated: ABC → ABBC

  • 12345 → 123345 (3 repeated)
  • HELLO → HELLLO (L repeated)
  • AB-1234 → AB-12234 (2 repeated)

How to recognize it

  • String is one character longer than original
  • Look for adjacent identical characters
  • One character appears twice in sequence

Common mistakes

  • Confusing with addition of different character
  • Missing the duplicate in longer strings
  • Not noticing legitimate double letters

Step-by-step walkthrough

Find the error: Original 'HELLO' vs Given 'HELLLO'

  1. Check lengths: original has 5 characters, given has 6
  2. Compare position 1: 'H' = 'H'
  3. Compare position 2: 'E' = 'E'
  4. Compare position 3: 'L' = 'L'
  5. Compare position 4: 'L' = 'L'
  6. Compare position 5: 'O' vs 'L' - extra 'L' found
  7. The letter 'L' was typed twice at position 4

Answer: Repeated character error: 'L' typed twice, creating 'LLL' instead of 'LL'

Learn 9

Reversal

A sequence of 3 or more adjacent characters is reversed.

Reversal errors occur when a portion of the string (typically 3-4 characters) is written in reverse order. This can happen when transcribing data from memory or when the eye jumps back while reading. Unlike simple transposition which swaps just two characters, reversal affects a longer sequence.

Substring ABC becomes CBA within the string

  • 123456 → 132456 (123 reversed to 321, shown as 132)
  • ABCDEF → ACBDEF (BCD reversed)
  • AB-1234 → AB-3214 (1234 partially reversed)

How to recognize it

  • Three or more characters appear in reverse order
  • Length remains the same
  • All original characters are present

Common mistakes

  • Confusing with multiple transpositions
  • Not recognizing partial reversals
  • Missing the reversal pattern in mixed data

Step-by-step walkthrough

Find the error: Original 'AB-1234' vs Given 'AB-3214'

  1. Compare prefix: 'AB-' = 'AB-'
  2. Compare position 4: '1' vs '3' - mismatch
  3. Compare position 5: '2' = '2'
  4. Compare position 6: '3' vs '1' - mismatch
  5. Compare position 7: '4' = '4'
  6. Pattern identified: '123' was reversed to '321'
  7. This is a 3-character reversal, not simple transposition

Answer: Reversal error: sequence '123' reversed to '321' (positions 4-6)

Learn 10

Spacing Error

An unwanted space is inserted into the string.

Spacing errors occur when the spacebar is accidentally pressed during data entry, inserting an unwanted space character. This is common when typing quickly or when hands drift on the keyboard. The resulting string is one character longer with a space where none should exist.

Space inserted at position N: ABCD → AB CD

  • 12345 → 123 45 (space inserted)
  • HELLO → HEL LO (space in middle)
  • AB-1234 → AB- 1234 (space after hyphen)

How to recognize it

  • String contains unexpected space character
  • Length is one character longer
  • Space appears where data should be continuous

Common mistakes

  • Overlooking spaces in visual inspection
  • Confusing with intentional formatting
  • Missing spaces at the start or end

Step-by-step walkthrough

Find the error: Original '12345' vs Given '123 45'

  1. Check lengths: original has 5 characters, given has 6
  2. Compare position 1: '1' = '1'
  3. Compare position 2: '2' = '2'
  4. Compare position 3: '3' = '3'
  5. Compare position 4: '4' vs ' ' - space found
  6. An unwanted space was inserted after position 3
  7. The remaining characters '45' are shifted by one position

Answer: Spacing error: unwanted space inserted between '3' and '4' at position 4

Learn 11

Format Shift

A separator or delimiter is moved to an incorrect position.

Format shift errors occur when a separator character (like a hyphen or dash) is placed in the wrong position within a formatted code. This changes the visual grouping of the data without changing the actual characters. It's common when manually entering codes that follow specific format patterns.

Separator moves from position M to position N: AB-1234 → A-B1234

  • AB-1234 → A-B1234 (hyphen shifted left)
  • AB-1234 → ABC-234 (hyphen shifted right)
  • 12-34-56 → 123-4-56 (hyphen repositioned)

How to recognize it

  • Separator in unexpected position
  • Same characters but different grouping
  • Total length remains the same

Common mistakes

  • Assuming format is always correct
  • Not checking separator positions
  • Focusing only on alphanumeric characters

Step-by-step walkthrough

Find the error: Original 'AB-1234' vs Given 'A-B1234'

  1. Compare position 1: 'A' = 'A'
  2. Compare position 2: 'B' vs '-' - mismatch
  3. Compare position 3: '-' vs 'B' - mismatch
  4. The hyphen moved from position 3 to position 2
  5. All characters are present but separator is misplaced
  6. Original format: 'AB-1234' (2 letters, hyphen, 4 digits)
  7. Error format: 'A-B1234' (1 letter, hyphen, 1 letter, 4 digits)

Answer: Format shift error: hyphen moved from position 3 to position 2

Learn 12

Phonetic Substitution

A character is replaced by a similar-sounding character.

Phonetic substitution errors occur when characters that sound similar are confused with each other. This is common when transcribing spoken data or working from dictation. Examples include C/K, F/V, M/N, B/P, D/T, and G/J confusions.

Common pairs: C↔K, F↔V, M↔N, B↔P, D↔T, G↔J

  • PACK → BACK (P confused with B)
  • CAME → KAME (C confused with K)
  • FIVE → VIVE (F confused with V)

How to recognize it

  • Character replaced with phonetically similar letter
  • Length remains the same
  • Often involves voiced/unvoiced consonant pairs

Common mistakes

  • Not considering phonetic relationships
  • Assuming spelling is always phonetic
  • Missing subtle sound-alike substitutions

Step-by-step walkthrough

Find the error: Original 'PACK' vs Given 'BACK'

  1. Compare position 1: 'P' vs 'B' - mismatch found
  2. Compare position 2: 'A' = 'A'
  3. Compare position 3: 'C' = 'C'
  4. Compare position 4: 'K' = 'K'
  5. The letter 'P' was replaced with 'B'
  6. These are phonetically similar: both are bilabial stops
  7. Common when transcribing from spoken dictation

Answer: Phonetic substitution error: 'P' confused with similar-sounding 'B' at position 1

Learn 13

Case Flip

A letter's case is changed from uppercase to lowercase or vice versa.

Case flip errors occur when the Caps Lock key is accidentally engaged or the Shift key is pressed/released at the wrong time. This changes a letter from uppercase to lowercase or vice versa. While the letter identity remains correct, the case change can cause issues in case-sensitive systems.

Uppercase letter becomes lowercase (A→a) or lowercase becomes uppercase (a→A)

  • HELLO → HeLLO (E became lowercase)
  • AB-1234 → Ab-1234 (B became lowercase)
  • CODE → COdE (D became lowercase)

How to recognize it

  • Same letter but different case
  • Length remains the same
  • Often occurs at word boundaries or after spaces

Common mistakes

  • Not checking case in case-sensitive contexts
  • Assuming all uppercase data is uniform
  • Missing case errors in mixed-case strings

Step-by-step walkthrough

Find the error: Original 'HELLO' vs Given 'HeLLO'

  1. Compare position 1: 'H' = 'H' (both uppercase)
  2. Compare position 2: 'E' vs 'e' - case mismatch found
  3. Compare position 3: 'L' = 'L' (both uppercase)
  4. Compare position 4: 'L' = 'L' (both uppercase)
  5. Compare position 5: 'O' = 'O' (both uppercase)
  6. The uppercase 'E' became lowercase 'e'
  7. Likely caused by Shift key release at wrong time

Answer: Case flip error: uppercase 'E' changed to lowercase 'e' at position 2

Learn 14

Rule Validation

Checking whether a record conforms to a set of format, range, type, or business rules.

Rule validation is the process of confirming that a piece of data satisfies explicit constraints before it is accepted. Typical rule classes are: (a) format rules — the value matches a pattern like an email, phone number, or date; (b) range rules — the value falls within allowed minimum and maximum bounds; (c) type rules — the value is a number, string, or boolean as expected; (d) business rules — domain-specific invariants (e.g., the total of line items equals the invoice amount). A record is valid iff it passes every applicable rule; the error is the first rule it fails.

valid(record) ⇔ ∀ r ∈ rules : r(record) holds

  • Email format: '[email protected]' ✓; 'user@@example' ✗
  • Age range [0, 130]: 25 ✓; 200 ✗
  • Business rule: invoice total must equal sum of line items

How to recognize it

  • A record is presented with a set of stated rules or constraints
  • The task asks whether the record is valid, or which rule it violates
  • Rules may combine (all must pass) or be hierarchical (some required, some optional)

Common mistakes

  • Stopping at the first rule that passes instead of checking every applicable one
  • Confusing 'valid format' with 'correct value' (a well-formed email can still be wrong)
  • Ignoring case sensitivity, leading/trailing spaces, or locale-specific separators

Worked examples

The worked examples below preserve the original practice set at every difficulty. Open an example to compare the prompt, answer choices, correct answer, and explanation.

Explanation

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