Human Error in Proofreading for Spelling Errors

Ray Panko's Human Error Website


Human error has been studied for more than 100 years. In the 1980s, there was a kind of Grand Unification, when human error researchers in different fields realized that their error rates were similar for cognitive tasks of similar complexity. There are now five basic truths that we know about human error.

An obvious error detection activity is proofreading. Proofreading research has traditionally focused on spelling errors rather than on grammar errors, which are more ambiguous. Spelling errors, furthermore, are divided into word and nonword errors. In nonword errors, the spelling error produces a text string that is not a dictionary word. For example, a spelling error might turn there into ther. In a word error, the mistake turns the word into another dictionary word. For example, there might become their.

Detection Rates for Word and Nonword Errors

Table 1 gives basic results from experiments in which subjects attempt to detect word errors, nonword errors, or both. The simple take-away is that participants caught 81% of the nonword errors but only 66% of the word errors. This result may be surprising to most people, who typically believe that proofreaders find nearly all errors. In fact, they do not, even if they are professionals. Note also that while spell-checking programs are nearly perfect at finding nonword errors, they offer almost no help for word errors. They are good what we are good at and terrible at what we are bad at.

Table 1: The Detection Rates for Word versus Nonword Spelling Errors

Source Word or Nonword Error Detection Rate Word Nonword
Cohen, G. [1980] Both 76%-94%    
Daneman & Stainton [1993] Nonword 89%   89%
Word 76% 76%  
Levy, Di Persio, & Hollingshead [1992] Nonword 86%-87%   87%
Word 69% 69%  
Riefer [1991] Nonword Simple Material 84%-86%   85%
Nonword Difficult Material 47%-58%   53%
Word 42%-62% 52%  
Wallace [1991] Nonword 94%   94%
Weighted Mean     66% 81%

Success Factors

Table 2 shows more details from these experiments. Among the results were:

Table 2: More Detail from Proofreading Studies

Source Word or Nonword Error Condition Detection Rate
Cohen, G. [1980] Both   76%-94%
Daneman & Stainton [1993] Nonword Own after 20 min 81%
Other after 20 min 89%
Own after 2 wks 88%
Other after 2 wks 91%
Word Own after 20 min 59%
Other after 20 min 76%
Own after 2 wks 68%
Other after 2 wks 83%
Healey [1980] Nonword 2 characters 92%
3 99%
4 94%
5 77%
6-10 72%
Levy & Begin [1984] Nonword Simple Material 83%
Difficult Material 86%
Levy, Di Persio, & Hollingshead [1992] Nonword   86%-87%
Word   69%
Riefer [1991] Nonword Simple Material 84%-86%
Difficult Material 47%-58%
Word   42%-62%
Spirrison & Gordy [1993] Unknown   76%
Wallace [1991] Nonword Vivid Imagers 93.5%
Poor Images 91.2%

Professional Proofreaders

Do professional proofreaders do better? The author has collected data from the last three editions of his networking and security textbook. This data includes the number of errors discovered in proofreading and afterward. Here, proofreading involved both spelling and grammatical errors. The percentage of errors discovered in this proofreading has averaged almost exactly 87%.


Cohen, G. (1980). Reading and Searching for Spelling Errors. In U. Frith (Ed.), Cognitive Processes in Spelling (pp. 135-155). London: Academic Press.

Daneman, M., & Stainton, M. (1993). The Generation Effect in Reading and Proofreading: Is it Easier or Harder to Detect Errors in One's Own Writing? Reading and Writing: An Interdisciplinary Journal, 5, 297-313.

Healey, A. F. (1980). Proofreading Errors on the Word The: New Evidence on Reading Units. Journal of Experimental Psychology: Human Perception and Performance, 6(1), 45-57.

Levy, B. A. (1983). Proofreading Familiar Text: Constraints on Visual Processing. Memory & Cognition, 11(1), 1-12.

Levy, B. A., & Begin, J. (1984). Proofreading Familiar Text: Allocating Resources to Perceptual and Conceptual Processing. Memory & Cognition, 12(6), 621-632.

Levy, B. A., Di Periso, R., & Hollingshead, A. (1992). Fluent Reading: Repetition, Automaticity, and Discrepancy. Journal of Experimental Psychology: Learning, Memory, and Cognition, 18(5), 957-971.

Riefer, D. M. (1991). Behavior Engineering Proposals: 4. "Is 'Backwards Reading' an Effective Proofreading Strategy?" Perceptual and Motor Skills, 73, 767-777.

Spirrison, C. L., & Gordy, C. C. (1993). The Constructive Thinking Inventory and Detecting Errors in Proofreading. Perceptual and Motor Skills, 76, 631-634.

Wallace, B. (1991). Imaging Ability and Performance in a Proofreading Task. Journal of Mental Imagery, 15(3&4), 177-188.