Error Guessing
Error guessing is an experience-based test technique in which testers design test cases based on their intuition, knowledge, and previous experience with similar defects.
“A test technique in which tests are derived on the basis of the tester's knowledge of past failures, or general knowledge of failure modes.”
(Definition reproduced from the ISTQB Glossary. Copyright belongs to ISTQB.)
View the complete ISTQB GlossaryWhat is Error Guessing?
Instead of following formal test design techniques, testers anticipate where defects are likely to occur and create targeted tests.
Experience driven: Testers rely on knowledge of common mistakes.
Flexible approach: Complements structured test design techniques.
Defect focus: Effective for uncovering unexpected failures.
Real World Example
A tester has noticed, across several past projects, that date-picker fields often break when a user selects February 29 on a leap year, or enters a date far in the past.
The formal requirements document for a new trip-booking form says nothing about leap years or unusual dates, so a purely specification-based test design would never generate a case for February 29 — it simply isn't mentioned anywhere.
Applying Error guessing, the tester draws on that personal experience and a mental checklist of common failure modes — empty fields, extreme dates, leap years, timezone edge cases — and deliberately books a trip departing February 29, 2028, even though no requirement calls it out.
The form crashes trying to calculate a return date exactly one year later, since February 29, 2029 doesn't exist — a defect that no specification-based technique would have found, because it lives precisely in the gap between what the requirements documented and what experienced testers know tends to break.
Practice Questions
Question 1
A tester, without any requirement mentioning it, deliberately tests a booking form using February 29 on a leap year because past experience shows date-picker fields often break on that date. Which technique is being applied?
Question 2
Which source of information is error guessing MOST directly based on?
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