The essential advantage of a distributed test group mirrors the biggest drawback to centralization: when resources are dedicated to a particular application or line of business, subject matter expertise is increased. Testers are more knowledgeable about what might otherwise be arcane or complex application behaviors, enabling them to improve testing coverage and effectiveness.
This model also fosters closer working relationships between development and test as team members interact more frequently and over longer time periods. Patterns and protocols develop that improve efficiencies between the groups, so that expectations are generally better understood and managed.
In this model the testers usually report to the project manager or to the application owner, and as such may be a peer to the developers on their team at the project level. However, at the department level the test organization itself is more likely to be part of development. As a result, they may be viewed as stepchildren or subordinates with less say about budget and schedules.
The major disadvantage of a distributed test organization is also the same as the biggest advantage of centralization: size. Specialization is less likely within a smaller team, and career paths are more likely to be limited. It is also harder to smooth out the peaks and valleys of resource demands without a larger pool to draw from. Processes tend to be more localized, with less consistency across application groups. This may result in either redundancy or conflict, especially when applications must be integrated.
The ideal strategy may be to combine the best of both. Centralize the functions that gain from economy of scale: specialization in tools, best-practices development and training, test environment provisioning, code migration, and release management procedures. But for subject-matter expertise, it may be more effective to draw from application experts who stay with the software across multiple projects and develop deep knowledge and relationships.
One way to accomplish this bifurcation is to align the QA/test organization centrally, but adopt a distributed user acceptance group. The advantage of this approach is that it provides career paths for everyone: QA/test professionals from IT can advance through a centralized organization by gaining specialized skills, and user acceptance testers can rise through the business ranks from end user to tester to systems or business analyst by gaining subject matter expertise.
Balance the Hierarchy
Whatever the organizational structure, remember that the reporting hierarchy plays a crucial role. Testers must be in control of critical decisions that affect their work, and their value to the overall corporation must be clear to upper management. Responsibility without authority is a formula for frustration and failure.