Two-Value Boundary Value Analysis (Exam D – Question 21)
1. Introduction
In the ISTQB Foundation Level exam, Boundary Value Analysis (BVA) is a black-box test technique used to test values at the edges of ranges.
This question asks you to use two-value BVA to achieve the best boundary coverage.
ISTQB에서 경계값 분석(BVA)은 범위의 경계(끝값)를 중심으로 테스트를 설계하는 대표적인 블랙박스 기법입니다.
이 문제는 2-값 경계값 분석(two-value BVA)으로 가장 높은 경계 커버리지를 제공하는 입력 조합을 고르는 문제입니다.
2. Practice Question
❓ Question
You are testing a temperature control system. The input is temperature in full degrees Celsius.
- If temperature is between 0 and 2 inclusive → display “temperature OK”
- If lower than 0 → display “temperature too low”
- If higher than 2 → display “temperature too high”
Using two-value boundary value analysis, which set of test inputs provides the highest level of boundary value coverage?
- a) –1, 3
- b) 0, 2
- c) –1, 0, 2, 3
- d) –2, 0, 2, 4
✅ Correct Answer: c) –1, 0, 2, 3
3. Explanation
Step 1: Identify equivalence partitions
Based on the specification, the input domain can be split into three partitions:
- Too low: { …, –2, –1 }
- OK: { 0, 1, 2 }
- Too high: { 3, 4, … }
요구사항에 따라 동등 분할은 3개입니다:
- 너무 낮음: 0 미만
- 정상(OK): 0~2 (포함)
- 너무 높음: 2 초과
Step 2: Identify boundary values for two-value BVA
For two-value BVA, you want to cover boundary values at the edges of partitions. The boundaries here are between:
- Too low ↔ OK at –1 / 0
- OK ↔ Too high at 2 / 3
So the boundary values to cover are:
- –1 (just below OK range)
- 0 (lower bound of OK range)
- 2 (upper bound of OK range)
- 3 (just above OK range)
2-값 BVA는 각 경계에서 양쪽 값을 선택합니다.
- 0 경계: –1과 0
- 2 경계: 2와 3
따라서 필요한 경계값은 –1, 0, 2, 3입니다.
✔ c) –1, 0, 2, 3 — Correct
Option c includes all four key boundary values, providing the highest boundary coverage.
선택지 c는 필요한 경계값 4개를 모두 포함하므로 경계값 커버리지가 가장 높습니다.
Why the other options are incorrect
❌ a) –1, 3
This covers values just outside the OK range, but it misses the exact boundaries 0 and 2.
❌ b) 0, 2
This covers only the boundaries inside the OK range, but it misses the adjacent outside values –1 and 3.
❌ d) –2, 0, 2, 4
This includes 0 and 2, but it does not include the closest outside boundary values (–1 and 3), which are required for two-value BVA.
a, b, d는 경계 양쪽 값을 모두 포함하지 못하므로 2-값 BVA 기준에서 부족합니다.
4. Summary Table
| Boundary | Partition Left | Partition Right | Two-Value Inputs | 의미 |
|---|---|---|---|---|
| 0 | Too low | OK | –1, 0 | 경계 양쪽 |
| 2 | OK | Too high | 2, 3 | 경계 양쪽 |
5. Final Takeaway
For the ISTQB exam, remember:
Two-value BVA selects values on both sides of each boundary. Here, that means –1 and 0 around the lower boundary, and 2 and 3 around the upper boundary.
시험 대비 핵심 문장입니다.
2-값 경계값 분석은 각 경계의 양쪽 값을 고른다. 이 문제에서는 –1/0, 2/3이므로 정답은 –1, 0, 2, 3이다.
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