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Common exam pitfall
Students often provide only the valid boundary value and omit the adjacent invalid value when asked for boundary test data.
Testing is the systematic execution of a program with curated test data to uncover syntax, logic, and runtime errors before software is deployed to users.
1. The Four Categories of Cambridge Test Data
Consider a validation rule requiring a student's examination score to be an integer between 0 and 100 inclusive ($0 \le \text{Mark} \le 100$):
Test Data Category | Definition | Example Values ($0 \le \text{Mark} \le 100$) | Expected System Behavior |
|---|---|---|---|
Normal | Data that is completely valid and lies comfortably within the expected range. |
| Accepted and processed normally. |
Extreme | Valid data situated on the exact upper and lower boundary limits. |
| Accepted as valid boundary values. |
Boundary | The pair of values on either side of each boundary limit (one valid extreme and one immediate adjacent invalid). | Lower limit: | Extreme value accepted; neighboring value rejected. |
Erroneous (Abnormal) | Data that falls completely outside allowable limits, has the wrong data type, or violates format rules. |
| Rejected with a clear error prompt. |
2. Why Boundary Testing is Essential
Programmers frequently introduce off-by-one errors when writing conditional operators (e.g. accidentally typing > instead of >=):
// Flawed boundary condition:
IF (Mark > 0) AND (Mark < 100) THEN ...- A normal test value like
50will pass this flawed code without revealing the bug. - Only testing the extreme boundary values (
0and100) reveals the error, because the flawed code erroneously rejects legitimate scores of0%and100%!
3. Structure of a Formal Test Plan
In Cambridge Paper 2, candidates are frequently asked to construct or complete a test plan table:
Test Case ID | Test Data Input | Test Data Type | Reason for Test | Expected Outcome |
|---|---|---|---|---|
|
| Normal | Typical valid percentage mark | Accepted and grade calculated |
|
| Extreme / Boundary | Minimum valid allowable score | Accepted |
|
| Boundary / Erroneous | Immediate value below lower threshold | Rejected with error message |
|
| Extreme / Boundary | Maximum valid allowable score | Accepted |
|
| Boundary / Erroneous | Immediate value above upper threshold | Rejected with error message |
|
| Erroneous | Non-numeric string data type | Rejected with type error message |
Independent test plan
A whole-number booking size must be from 2 to 9. Propose one normal test, both extreme tests, the two boundary pairs and an abnormal type test. Write expected acceptance/rejection for each before running anything. Compare: 5 accepts; 2 and 9 accept; 1/2 and 9/10 exercise both sides of each boundary; text such as “many” is rejected. Other normal values within the range are valid choices.
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Exam-Style Check: Classify Boundary Test Data Pairs
• Multiple ChoiceAn algorithm validates an exam percentage mark that must be between 0 and 100 inclusive. Which set of numbers represents a complete set of boundary test data for the upper limit?
Interactive exercises for Module 2 and beyond require an active plan.