Why Test coverage metrics in Testing Fundamentals? - Purpose & Use Cases

The Big Idea

What if you could instantly know which parts of your software are still risky and untested?

The Scenario

Imagine you have a huge book to proofread, but you only have a checklist of chapters without any details on which pages or paragraphs you checked.

You try to remember what you read, but it's easy to miss some parts.

The Problem

Manually tracking which parts of software are tested is slow and confusing.

You might think you tested everything, but some code lines or features remain unchecked.

This leads to bugs slipping into the final product.

The Solution

Test coverage metrics automatically measure which parts of the code are tested.

This gives clear numbers and reports showing what is covered and what is not.

It helps testers focus on missing tests and improve quality.

Before vs After

✗ Before

Checked features: login, signup, profile update
No details on code lines tested

✓ After

Test coverage: 85% lines covered
Missing tests: password reset, email verification

What It Enables

Test coverage metrics make it easy to see gaps in testing and ensure software is well checked before release.

Real Life Example

A team uses test coverage reports to find untested code paths causing crashes in their app, then writes tests to fix those gaps.

Key Takeaways

Manual tracking of tested code is unreliable and slow.

Test coverage metrics give clear, automatic insight into testing completeness.

They help improve software quality by highlighting missing tests.

Practice

1. What does test coverage metric measure in software testing?

easy

A. The time taken to run all tests

B. The number of bugs found during testing

C. The percentage of code or features tested compared to total code or features

D. The number of testers involved in the project

Solution

1. Step 1: Understand the purpose of test coverage

   Test coverage measures how much of the software is tested, usually as a percentage.

2. Step 2: Identify what is compared in coverage

   It compares tested code or features against the total code or features available.

3. Final Answer:

   The percentage of code or features tested compared to total code or features -> Option C

4. Quick Check:

   Test coverage = tested items / total items [OK]

Hint: Coverage shows tested parts over total parts [OK]

Common Mistakes:

• Confusing coverage with number of bugs
• Thinking coverage measures test duration
• Assuming coverage counts testers

2. Which of the following is the correct formula for calculating test coverage percentage?

easy

A. (Number of tested code lines / Total code lines) * 100

B. (Total tests / Number of testers) * 100

C. (Number of bugs found / Total tests) * 100

D. (Number of tests run / Total number of tests) * 100

Solution

1. Step 1: Identify what test coverage measures

   Test coverage measures how much code is tested, so it relates to tested code lines over total code lines.

2. Step 2: Match formula to coverage concept

   The formula is (tested code lines / total code lines) * 100 to get percentage coverage.

3. Final Answer:

   (Number of tested code lines / Total code lines) * 100 -> Option A

4. Quick Check:

   Coverage % = tested lines / total lines * 100 [OK]

Hint: Coverage = tested code lines divided by total lines [OK]

Common Mistakes:

• Using number of tests run instead of code lines
• Confusing bugs found with coverage
• Dividing by number of testers

3. Consider a software module with 200 lines of code. If 150 lines are executed during testing, what is the test coverage percentage?

medium

A. 85%

B. 75%

C. 65%

D. 70%

Solution

1. Step 1: Identify total and tested lines

   Total lines = 200, tested lines = 150.

2. Step 2: Calculate coverage percentage

   Coverage = (150 / 200) * 100 = 0.75 * 100 = 75%.

3. Final Answer:

   75% -> Option B

4. Quick Check:

   150/200 = 0.75 = 75% [OK]

Hint: Divide tested lines by total lines, multiply by 100 [OK]

Common Mistakes:

• Dividing total lines by tested lines
• Forgetting to multiply by 100
• Mixing up tested and total lines

4. A tester reports 90% test coverage, but some critical code paths are not tested. What is the likely issue?

medium

A. Coverage metric only measures quantity, not quality of tests

B. Tester calculated coverage incorrectly

C. Test coverage always guarantees all paths tested

D. The software has no untested code

Solution

1. Step 1: Understand what coverage measures

   Coverage measures how much code is tested but not which specific paths or quality of tests.

2. Step 2: Identify why critical paths might be missed

   High coverage can still miss important paths if tests do not cover all scenarios.

3. Final Answer:

   Coverage metric only measures quantity, not quality of tests -> Option A

4. Quick Check:

   Coverage % ≠ test quality [OK]

Hint: Coverage shows quantity, not test quality [OK]

Common Mistakes:

• Assuming coverage means all paths tested
• Blaming tester calculation without proof
• Ignoring untested critical paths

5. You have a project with 3 modules: Module A (100 lines, 80 tested), Module B (150 lines, 120 tested), Module C (50 lines, 40 tested). What is the overall test coverage percentage?

hard

A. 70%

B. 75%

C. 77.5%

D. 80%

Solution

1. Step 1: Calculate total lines and total tested lines

   Total lines = 100 + 150 + 50 = 300 lines. Tested lines = 80 + 120 + 40 = 240 lines.

2. Step 2: Calculate overall coverage percentage

   Coverage = (240 / 300) * 100 = 80%.

3. Step 3: Recalculate carefully

   Sum tested lines: 80 + 120 + 40 = 240. Sum total lines: 100 + 150 + 50 = 300. Coverage = (240/300)*100 = 80%.

4. Final Answer:

   80% -> Option D

5. Quick Check:

   Total tested / total lines = 240/300 = 80% [OK]

Hint: Sum tested and total lines, then divide and multiply by 100 [OK]

Common Mistakes:

• Averaging percentages instead of totals
• Adding percentages directly
• Mixing tested and total lines