Synopsis

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.

We are nearly perfect when we work, making errors only about 1% to 5% of the time.
However, when many actions must be done correctly in sequence, this tiny error rate balloons into a high probability of making an error.
We are much worse at detecting errors; we are only 40% to 90% successfulat finding an error when we work alone.
Due to the way our cognition works, we make many more errors than we believe we do, we badly overestimate our efficacy in finding errors, and our poor error intuition should cause us to reject prescriptions for reducing errors until they have been empirically determined to be safe and effective.

On this page, we look at error rates in such trivial cognitive actions such as turning a valve and in such simple but nontrivial actions such as writing a line of code, writing a sentence, or entering a formula in a spreadsheet.

Errors in Trivial and Simple but Nontrivial Cognitive Actions

Table 1 shows error rates in trivial and in simple nontrivial cognitive actions. The table indicates that error rates in simple but nontrivial activities typically average 1% to 5%. Programming and spreadsheet error rates are near the high end of this range. For more complex cognitive actions, error rates, as you would suspect, are higher.

Table 1: Error Rates for Simple Trivial and Nontrivial Actions

Category	Description	Error Rate	Study
Trivial	Entering a mail code	0.5%	Baddeley & Longman [1973]
Trivial	Flash one of two letters on screen; subject hits one of two buttons	0.6%	Rabbit [1990]
Trivial	Residual typing errors per character.	0.6%	Shaffer & Hardwick [1968]
Trivial	Five-digit addition	0.7%	Trumbull [1952a]
Trivial	Error in routine action when care is required	1.0%	Kirwan [1994]
Trivial	Measuring and calculating the size of small holes	1.0%	Rigby & Edelman [1986]
Trivial	Enter number into a calculator	1.0%	Smith [2005]
Trivial	Record information on graph incorrectly	1.0%	Smith [2005]
Trivial	Simple arithmetic calculation with or without a calculator	1.0%	Swain & Guttman [1983]
Trivial	Error reading a graph. Per read.	1.0%	Swain & Guttman [1983]
Nontrivial	Grammatical errors per word	1.1%	Chedru & Geschwind [1972]
Nontrivial	Calculator tasks . Per multipart calculation.	1.5%	Melchers & Harrington [1982]
Nontrivial	Students performing table lookup tasks. Per lookup.	1.5%	Melchers & Harrington [1982]
Nontrivial	Five-digit multiplication	1.5%	Weldon [1956]
Nontrivial	Undergraduate spelling and grammar errors, per word	1.6%	Hotopf [1980]
Nontrivial	Five-digit subtraction	1.6%	Trumbull [1952b]
Nontrivial	Error in doing simple arithmetic	2.0%	Smith [2005]
Nontrivial	Writing a software statement. National Software Quality Experiment	2.0%	O'Neill [1994]
Nontrivial	Errors in high school essays, per word	2.4%	Mitton [1987]
Nontrivial	Writing a software statement. Litton. 150 inspections	2.6%	Madachy [1996]
Nontrivial	Simple arithmetic error with self-checking	3.0%	WASH-1400
Nontrivial	Writing a software statement. 2,500 inspections at Cisco Systems	3.2%	Cohen [2006]
Nontrivial	Writing a software statement. ATT. 2.5 million lines of code over 8 software releases	3.7%	Graden & Horsley [1986]
Nontrivial	14 laboratory studies of spreadsheet development, 967 participants working alone on a variety of tasks	3.9%	Spreadsheet Research Website
Nontrivial	Put 10 digits into a calculator	5.0%	Smith [2005]

References

Baddeley, A. D., & Longman, D. J. A. (1978). The Influence of Length and Frequency of Training Session on the Rate of Learning to Type. Ergonomics, 21(8), 627-635.

Chedru, F., & Geschwind, N. (1972). Writing Disturbances in Acutely Confusional States. Neuropsychologia, 10, 343-353.

Cohen, Jason (2006). Best Kept Secrets of Peer Code Review, Austin Texas: Smart Bear, Inc.

Fagan, M. E. (1976). Design and code inspections to reduce errors in program development, IBM Systems Journal, 15(3), 182-211.

Fagan, M.E. (1986, July). Advances in software inspections, IEEE Transactions on Software Engineering, Vol. SE-12, No. 7, Page 744-751.

Graden, M., & Horsley, P. (1986). The Effects of Software Inspection on a Major Telecommunications Project. AT&T Technical Journal, 65.

Hotopf, N. (1980). Slips of the Pen. In Frith (Ed.), Cognitive Processes in Spelling (pp. 287-307). London: Academic Press.

Madachy, R. J. (1996, July) "Measuring Inspection at Litton," Software Quality, 2(4) pp. 1-10.

Melchers, R. E., & Harrington, M. V. (1982). Human Error in Simple Design Tasks (Civil Engineering Research Reports Report Number 31): Monash University.

Mitton, R. (1987). Spelling Checkers, Spelling Correctors, and the Misspellings of Poor Spellers. Information Process Management, 23(5), 495-505.

O'Neill, Don, (1994, October) "National Software Quality Experiment," 4th International Conference on Software Quality Proceedings.

Rigby, L. V. and A. D. Swain, "Effects of Assembly Error on Product Acceptability and Reliability," pp 3-12 to 3-19 in Proceedings of the 7th
Annual Reliability and Maintainability Conference, American Society of
Mechanical Engineers, New York, July, 1968.

Rabbit, P. (1990). Age, IQ and Awareness and Recall of Errors. Ergonomics, 33(10/11), 1291-1305.

Swain, A. D., & Guttman, H. E. (1983). Handbook of Human Reliability Analysis with Emphasis on Nuclear Power Plant Applications Technical Report NUREG/CR-1278). Washington, D.C.: U. S. Nuclear Regulatory Commission.

WASH-14 (NUREG-75/014) "Reactor Safety Study -- An Assessment of Accident Risks in U.D. Commercial Nuclear Power Plants," 1975.

Weldon, R. J., Error Reductions in Computations, SC-4074(TR), Sandia National Laboratories, Albuquerque, NM, November 1956.

Trumbull, R., Analysis of Basic Factors Constituting Necessary Mathematical Proficiency Required for Success in Naval Aviation: Report I, Addition, Rpt. No. N.M. 001058.20.01, U.S. Naval School of Aviation Medicine, Naval Air Station, Pensacola, FL, 1952.(a)

Rigby, L. V. and A. D. Swain, "Effects of Assembly Error on Product Acceptability and Reliability," pp 3-12 to 3-19 in Proceedings of the 7th Annual Reliability and Maintainability Conference, American Society of Mechanical Engineers, New York, July, 1968.

Kirwan, B. A. (1994). A guide to practical human reliability assessment, London: Taylor & Francis.

Shaffer, L.H. & Hardwick, J. “Typing Performance as a Function of Text,” Quarterly Journal of Experimental Psychology (20:4) November 1986, pp. 203-214.