In software testing terminology, what is meant by parallel or audit testing?

Introduction / Context:
Parallel testing, also known as audit testing, is a technique used when a new system is introduced to replace an existing one. Instead of immediately decommissioning the old system, both systems run side by side for a period, processing the same inputs. The idea is to compare the outputs and behaviour of the new system against the trusted legacy system to build confidence that the new implementation is correct. This question asks you to identify the accurate description of parallel or audit testing.

Given Data / Assumptions:

• An organisation has an existing production system that has been in use for some time.
• A new system or a significantly upgraded version is being rolled out.
• There is a desire to minimise risk and verify that the new system behaves correctly before fully switching over.
• Both systems can access the same or replicated input data for comparison.

Concept / Approach:
In parallel or audit testing, both the old and new systems are operated simultaneously using identical or carefully matched input data. Testers and business users then compare the results produced by the two systems. If the outputs match within acceptable tolerances and the new system meets performance and usability criteria, confidence in the new implementation increases. If discrepancies appear, they can be analysed to determine whether the new system has defects or whether the old system contained hidden issues. This approach is especially useful in domains where correctness and regulatory compliance are critical, such as finance or healthcare.

Step-by-Step Solution:
Step 1: Identify that the term parallel refers to two systems running side by side. Step 2: Recognise that audit testing involves comparing the output of a new system against a reference system regarded as correct. Step 3: Understand that the key goal is to validate the new system by ensuring that it produces the same or better results as the legacy system when given the same inputs. Step 4: Evaluate option a, which describes running both systems in parallel on the same data and comparing outputs. Step 5: Reject other options that miss the idea of using an existing system as a reference point.

Verification / Alternative check:
Textbooks on software testing describe parallel testing as running the new application concurrently with the existing one so that results can be compared directly. Case studies in banking, for example, show organisations calculating balances and interest with both systems for a period and reconciling any differences. This is exactly what option a describes and is not consistent with the descriptions in the other options, which focus on unrelated testing activities.

Why Other Options Are Wrong:
Option b refers to running unit tests and performance tests together, which might occur in some pipelines but does not capture the idea of having two independent systems processing the same business data. Option c talks about multiple testers executing the same test cases, which is closer to double checking but not to parallel system testing. Option d limits the concept to licence compliance audits, which is a different type of audit and not what is meant by parallel or audit testing in the context of validating new applications.

Common Pitfalls:
A common pitfall is underestimating the operational overhead of running two systems in parallel, including data synchronisation, user training, and environment maintenance. Another mistake is failing to define clear acceptance criteria for what constitutes acceptable differences between the systems. For examinations, remember the core definition: parallel or audit testing means operating the new and old systems in parallel on the same inputs and comparing their outputs to validate the new system.

Final Answer:
Parallel or audit testing is running a new system and the existing legacy system in parallel on the same data and comparing outputs to validate correctness.