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Manual Testing · Fundamentals

Levels of Testing

Software is tested at several levels, from the smallest piece of code to the whole system in the customer’s hands.

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On this page · 5 min · Beginner

On this page

  1. Learning objectives
  2. Explanation
  3. Code example
  4. Step by step
  5. Real-world use
  6. Common mistakes
  7. Best practices
  8. Remember this
  9. Practice exercise
  10. Mini quiz
  11. Related topics
  12. My notes
Reading time
5 min
Difficulty
Beginner
Updated
Oct 29, 2026

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Interview questions

Learning objectives#

  • Name the four test levels
  • Know who performs each level
  • Compare integration approaches
  • Distinguish alpha, beta and UAT

Explanation#

Unit testing: individual functions or classes, usually by developers, often automated.

Integration testing: interactions between modules or services (top-down, bottom-up, big-bang). Finds interface and data-flow defects.

System testing: the complete, integrated application against requirements — functional and non-functional — by the QA team.

Acceptance testing: confirms the system meets business needs. Alpha (internal users), beta (real users in the field), UAT (customer sign-off).

Code example: Online food ordering app#

TextOnline food ordering app Illustrative
Unit         calculateTotal([{price: 200, qty: 2}], coupon "SAVE10") → 360
Integration  Cart service → Payment service: amount and currency passed correctly
System       Browse → add items → pay by UPI → order confirmed → SMS sent
Acceptance   Restaurant owner confirms orders appear on the kitchen tablet as agreed
Expected output
Each level finds different kinds of defects; together they cover the whole product.

This example is illustrative. Copy or download it and run it in your own environment; the expected output is shown above.

Step-by-step explanation#

  1. Unit tests check one calculation.
  2. Integration tests check two services talking.
  3. System tests check a full user journey.
  4. Acceptance tests confirm business expectations.

Real-world use case#

A QA team focuses on system testing while developers own unit tests and the client runs UAT before go-live.

Common mistakes#

Skipping integration testing

Modules that work alone often fail together because of mismatched data or contracts.

Treating UAT as the main bug hunt

UAT confirms readiness; most defects should be found earlier.

Best practices#

  • Test at every level
  • Use realistic data in system testing
  • Plan UAT scenarios with business users
  • Automate unit and integration tests

Remember this#

Unit → integration → system → acceptance.

Integration finds interface bugs.

Alpha in-house, beta in the field, UAT for sign-off.

Practice exercise#

Your task: Which level checks that the Orders API correctly calls the Inventory API?

Show solution
Text Illustrative
Integration testing.

Mini quiz#

Levels of Testing quizMultiple choice Question 1 of 1

Who typically performs UAT?

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My notes#

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