Test coverage is a useful metric – but a high percentage isn’t everything. Let’s take a closer look at what it really shows and the misunderstandings it can lead to if we rely on it too heavily.
What is test coverage?
Test coverage generally indicates how much of your code is executed during test runs. It can be measured in different ways (line, branch, or function coverage), but the percentage figure can often be misleading. A high coverage number may look good in a report, but does it truly mean our software is bug-free? In this article, we examine what test coverage actually means, why a 100% rate can be deceptive, and how to use this metric consciously to genuinely improve software quality.
Why is 100% coverage misleading?
Test coverage shows what parts of the source code were executed during tests. The most common types are:
- Line coverage: the percentage of code lines executed during tests.
- Branch coverage: shows whether conditional branches (if/else or switch cases) have been covered by tests.
- Function coverage: indicates which functions and logical paths were executed.
It’s important to understand: test coverage measures what was executed, not what was actually verified. Just because a line of code was run during a test doesn’t mean its behavior was properly checked! That’s why it’s worth using a reliable test coverage tool that ensures critical test cases are not missed and provides a comprehensive picture of the software’s true current coverage state.
Common pitfalls of chasing 100% coverage
A perfect score might look impressive in reports – but it can also create a false sense of security. When a team focuses solely on achieving maximum coverage, it can easily backfire:
- Shallow tests that run but don’t actually verify anything.
- Test cases written just to boost coverage stats, not to find real bugs.
- High-risk, harder-to-test parts of the code are neglected, while simple parts are over-tested
- Time is wasted on low-value code – like getters, setters, or boilerplate logic – which rarely contain real bugs.
If developers treat coverage as just another checkbox, it can weaken the team’s quality mindset over time.
How to use test coverage the smart way
Test coverage can be an incredibly helpful feedback tool – if used properly. Here’s how to make it a meaningful part of your workflow:
1. Don’t rely solely on automated testing
Automated tests are essential, especially for large codebases and regression testing. However, rare edge-case bugs and unexpected user behaviors still require manual testing.
One major advantage of TestNavigator is that it lets you manage both manual and automated tests in a single cycle – giving you a much clearer view of your software’s true coverage.
2. Focus on risk-based testing
Don’t aim to cover every line – focus on the logic where failure would cause the most impact. This includes:
- Business-critical processes
- Financial calculations
- Data transformations
- External service integrations
3. Prioritize quality verification over execution
Use real assertions, test realistic scenarios, and avoid chasing numbers for their own sake. Remember: users rarely behave "as expected." One typo in a date field could lead to unexpected software errors.
4. Prioritize what matters most
Take advantage of test prioritization! TestNavigator’s smart feature helps rank your test cases so the most important ones are run first, ensuring that critical areas don’t get overlooked.
The real goal
Test coverage is a valuable metric – if used in context. But 100% coverage doesn’t equal 100% software quality. The goal isn’t to chase perfect numbers! It’s to build stable, reliable, and well-tested software.