SIT vs UAT: What’s the difference?

In this blog post, we’ll talk about SIT (System Integration Testing) and UAT (User Acceptance Testing)—what they mean, how they differ, and why you need both before releasing software.

What is SIT (System Integration Testing)?

System Integration Testing (SIT) validates that every piece of your system—frontend, backend, APIs, databases, and third‑party integrations—works together as one ecosystem.

SIT process overview

• Preparation phase: define the SIT test plan (what to test and in what order); prepare sample data; set up a safe test environment; use placeholders (stubs/drivers) if parts aren’t ready.
• Execution phase: run test cases that mimic real-world user actions end‑to‑end (e.g., login → browse → checkout); verify data flow across subsystems; rerun tests after fixes (regression testing).
• Exit criteria: all planned integration cases executed; no critical/blocker defects; system stable for UAT.
• Deliverables: SIT plan, execution report, defect log. Ensures traceability for handoff to UAT.

What is UAT (User Acceptance Testing)?

User Acceptance Testing (UAT) is where the business and end users validate that the system meets their requirements and feels right to use. It answers, “Does this work for us?”

Read more: What is User Acceptance Testing (UAT)? Meaning, Definition, Process

UAT process overview

• Preparation phase: define acceptance criteria with stakeholders; prepare a staging environment; draft UAT scripts; train testers.
• Execution phase: business users perform real-world workflows; capture usability and functional feedback; log issues via scripts or tools (e.g., Marker.io).
• Exit criteria: critical workflows validated; major defects resolved; formal sign‑off.
• Deliverables: UAT scripts, feedback reports, sign‑off document.

SIT vs UAT: Key differences

Methodologies: SIT vs UAT

SIT methodologies

• Bottom-up approach: build up from low-level pieces (e.g., database → API → backend service → UI). Example: first confirm the checkout API saves orders correctly, then connect it to the backend cart, then to the frontend “Place order” button.
• Top-down approach: start from user-facing flows and stub missing lower layers. Example: drive the “Sign in → My account” journey in the UI while stubbing the user database to return test profiles until the real DB is ready.
• Hybrid: combine both to move fast without losing coverage. Example: test real payment → order systems bottom‑up, while top‑down stubbing email/SMS notifications that aren’t built yet.

UAT methodologies

• Scripted testing: testers follow step‑by‑step scripts mapped to acceptance criteria. Example: “As a sales manager, apply a 10% discount to a B2B order and verify the invoice total and dashboard report.”
• Exploratory testing: testers roam like real users to find edge cases and usability snags. Example: a customer success rep tries to edit an order from their phone and discovers the date picker blocks the keyboard.
• Beta testing: limited release to real users in near‑live conditions to validate fit and stability. Example: launch the new checkout for 5% of traffic and monitor errors and user feedback.

Why teams confuse SIT and UAT

Teams often confuse SIT and UAT because both phases happen late in the software development lifecycle, exercise end‑to‑end flows, and share the goal of catching issues before production.

The crucial distinction is perspective: SIT is QA‑led and system‑focused, while UAT is business requirements‑led and user perspective‑focused.

Why both matter (and what happens if you skip one)

Skipping either phase creates blind spots.

With SIT without UAT, the system can look technically sound while critical business workflows (like pricing rules or approval paths) still fail.

With UAT without SIT, users may sign off on happy paths while hidden integration defects surface in production.

Running both provides a reliable, high‑quality release that’s functional and user‑approved.

Final thoughts

System integration testing ensures your system works as a whole. User acceptance testing ensures it works for your users and your business.

They’re different, sequential, and complementary.

Want to simplify your UAT process? Try Marker.io to keep QA and UAT feedback flowing in one place.