Understanding Quality – Unit Testing and Acceptance Testing

Ramy Loaiza , Principal Testing Manager, Testlio

April 7th, 2025

The global software testing market was valued at USD 87.42 billion in 2024, and it is projected to grow to USD 512.3 billion by 2033, reflecting a CAGR of 21.71%. As digital experiences reshape how we consume services and goods, interact with each other, and share knowledge and value in a seamless, real-time way, ensuring their quality has become critical. 

When a digital experience fails to deliver quality and meet or exceed users’ expectations, money is lost, and brands—even those that may not live entirely in the digital ecosystem—become tarnished and damaged. 

Yet, despite this, many teams globally struggle to find the right balance in their testing strategy or even implement a QA/QE strategy. A lack of user input, poor preparation, and limited resources often lead to software that may function but fails to meet the user’s needs.

It can be challenging for teams to understand how different types of testing are part of a comprehensive QA/QE strategy and that only using a set of them when more are required can hamper quality at the end of the road. 

It’s still pretty familiar to hear development teams -at least in some regions like LATAM- say, “Hey, we don’t need any more testing beyond unit testing as we’re ensuring everything is good at the code level.”  

Sadly, that is not true. 

Sure, the code can be sound, and the core business logic can be covered by unit testing based on engineering specs, coding standards, and frameworks. However, it has been proven that even when code passes unit testing, it may fail to survive the user. This may be because of noticeable quality issues or bugs or because the scoped and coded user flows and features may not match the user expectations, be it because it doesn’t make sense to the end user or because things may be counterintuitive. 

Enter acceptance tests or user acceptance testing: UAT, as we love to call it in the industry. 

While unit tests ensure that the puzzle pieces fit together nicely and that, from a coding perspective, there are no rough edges, (user) acceptance tests validate that the finished product delivers the expected experience.

This article will explore the differences and similarities between unit and acceptance testing, examine their complementary roles, and discuss how teams can strike the right balance.

Unit vs. Acceptance Testing: Where They Belong in Your QA Plan

When it comes to creating a robust software testing strategy, understanding the roles and placement of different types of tests is crucial. 

Let’s explore how unit and acceptance testing fit the bigger picture.

Testing Pyramid Perspective

Think of your testing strategy like a pyramid. At the bottom, unit tests provide fast, targeted feedback on individual pieces of code. 

Moving up, integration tests check how different components work together. At the very top, acceptance tests ensure the whole system functions as expected from the user’s perspective.

A well-balanced approach includes:

• A set of unit tests that ensure the code matches the specifications and guidelines and catches business logic, resource allocation, system performance, and other issues and bugs that can be small at the design stage but become huge later. 
• A set of integration tests to check how different systems interoperate between themselves and that any kind of data input remains consistent if needed across flows, features, and potential usage scenarios.
• A set of acceptance tests focused on key end-to-end workflows.

This structure keeps development fast and efficient, preventing minor bugs from becoming big problems.

Shift-Left vs Shift-Right

Quality assurance and testing aren’t just about running the right types of tests—they’re also about running them at the right time. The best QA strategies follow the shift-left, shift-right approach.

Unit tests fit into shift-left testing and QA. They’re written early in development, helping developers catch issues before they snowball into more significant problems. These tests run fast and provide instant feedback so teams can iterate quickly. Functional tests can also be extrapolated from some unit testing cases (i.e., validation subroutines unit testing into form field functional validation), so the efforts are cascaded and coordinated from the beginning. 

Acceptance tests align with shift-right testing. They come in before release, ensuring the software works as expected in real-world scenarios. These tests are broader and slower but essential for validating that everything works.

Chaining efforts in those stages can ensure a healthy, quality release. At the end of the day, quality engineering (and assurance) can be understood as a mindset, a series of frameworks, milestones, indicators and goals that live and exist from the very beginning of the software development lifecycle till a viable release it’s out and beyond. 

Agile and DevOps

Modern development moves fast, and testing needs to keep up. Agile and DevOps teams rely on unit and acceptance testing to ship high-quality software without slowing down.

Unit tests are a developer’s first line of defense. They’re often written alongside code using Test-Driven Development (TDD) tools and systems and/or are included in Continuous Integration (CI) pipelines. If a unit test fails, the developer fixes the issue before it affects the rest of the system.

Acceptance tests focus on the big picture. These tests validate that new features meet user and business needs, often involving QA engineers, product managers, and sometimes real users.

Key Similarities and Differences

Unit testing prevents developers from introducing minor, critical bugs that could break functionality later. In contrast, acceptance testing prevents teams from releasing software that meets technical requirements but fails to satisfy actual needs.

Let’s break down where they align and where they differ to fully understand their role in a testing strategy and the difference between unit and integration testing.

Similarities

The following are unit and acceptance testing similarities that contribute to developing reliable software.

• Objective: Both types aim to identify issues and ensure software quality. They help teams detect problems early and reduce the number of bugs that reach production.
• Approach: Both unit and acceptance tests rely on clearly defined test cases, including structured documentation such as test steps, test data, and expected outcomes.
• Bug Prevention: Both are proactive methods to prevent defects from reaching later stages of software development, ultimately reducing development costs and effort.
• Automation Potential: Each type of testing can benefit from test automation, which can enhance efficiency, allow quicker feedback and consistent execution, and reduce the manual testing workload.
• Verification of Requirements: Both methodologies ensure the software behaves according to predefined expectations.

Differences

To help clarify when and how each testing type is best used, let’s break down some key differences between unit testing and acceptance testing. 

To put these differences into perspective, consider these examples:

Unit Testing Example:

Scenario: Validate that the calculateDiscount() method functions correctly.

Input: Price = 100, discount = 20%

Expected Result: The method returns 80.

Acceptance Testing Example:

Scenario: The User can successfully purchase an item.
Steps:

1. Log in with valid credentials.
2. Add a product to the shopping cart.
3. Proceed to checkout.
4. Complete payment using valid credit card details.

Expected Result: User receives an order confirmation and receipt; the inventory is updated accordingly.

Real-World Use Cases

Unit and acceptance testing are essential in every industry, but their application varies depending on specific challenges and priorities. 

In some fields, like finance and healthcare, accuracy is non-negotiable, while in industries like SaaS and e-commerce, the focus is on user experiences. 

Let’s explain how different industries use these tests to keep their software reliable and user-friendly.

SaaS

Subscription management and billing accuracy are critical for SaaS companies, from productivity like Microsoft to media and entertainment like Netflix. Customers expect hassle-free upgrades, downgrades, and renewals, and billing errors can lead to frustration or lost revenue. 

Unit tests ensure pricing calculations, discounts, and promotional offers work as intended. Since SaaS pricing models can be complex, automated unit testing helps developers roll out changes confidently without breaking existing subscriptions.

Acceptance tests focus on the full user experience of managing a subscription. They verify if a user can upgrade their plan without losing features. They also check if the system correctly generates invoices and sends confirmation emails.

E-commerce

In e-commerce, customers expect a smooth, error-free checkout process. A small mistake—like an incorrect price calculation or a glitch in payment processing—can lead to abandoned carts and lost sales.

Unit tests verify that pricing, tax calculations, shipping costs, and discount codes are applied correctly. These calculations happen in milliseconds, so they must be fast and accurate to prevent discrepancies between product listings and final checkout prices.

Acceptance tests check that the entire purchasing journey flows smoothly. From adding items to the cart to completing payment, these tests simulate real-world shopping experiences. 

They also help catch issues like failed transactions, stock mismanagement, or order confirmation delays.

Finance

In the financial sector, even a minor miscalculation can have significant consequences. Banks, payment platforms, and investment firms rely on software for interest rates, loan payments, and transaction processing.

Therefore, unit tests are necessary for verifying complex financial calculations and ensuring that interest rates, loan schedules, and fees are computed correctly. 

A small coding error in these calculations could lead to significant financial losses or legal issues.

On the other hand, acceptance tests validate end-to-end financial workflows, such as loan applications, fund transfers, and credit approvals. 

These tests confirm that users can submit documents, get approvals, and access funds without errors. 

Healthcare

Software errors in healthcare aren’t just inconvenient—they can be life-threatening. Whether managing patient records, scheduling appointments, or calculating medication dosages, healthcare software needs to be accurate, secure, and compliant with regulations like HIPAA and GDPR.

Unit tests validate critical medical calculations, such as medication dosages, patient risk scores, and diagnostic algorithms. 

These tests help prevent potentially dangerous mistakes, ensuring patients receive the proper treatment based on accurate data.

Acceptance tests can verify if a doctor can update a patient’s record without losing information and if the system correctly sends prescription details to a pharmacy.

These tests make sure that healthcare providers can rely on the software without disruptions that could impact patient care.

Final Thoughts

Unit tests should be fast and frequent, providing quick feedback during development. On the other hand, acceptance tests should focus on key workflows, validating that everything comes together as expected before release. 

If you place too much emphasis on one without the other, you may end up with software that technically works but does not meet users’ needs or software that meets business requirements but is fragile under the hood.

Testing strategies need to evolve alongside modern development practices like Agile and DevOps. Automation, continuous integration, and real-world testing all help teams ship better software faster. 

The more structured your testing approach, the fewer surprises you’ll encounter. If balancing unit and acceptance testing feels difficult, Testlio can help.

Whether you need reliable unit testing, real-world acceptance testing, or both, Testlio provides on-demand testing solutions that fit your development cycle.

With Testlio, you can:

• Get expert testers on real devices to validate your product in real-world conditions.
• Automate testing workflows for faster, more efficient test execution.
• Scale testing as needed, whether you’re launching a new feature or refining an existing one.

Learn more about how Testlio can support your team.