Writing requirements rationale is crucial because it explains the reasoning behind each requirement and why it is necessary for the project. This practice helps ensure that requirements are clear, consistent, and aligned with stakeholder needs. Additionally, it supports the creation of testable and achievable requirements within project constraints.
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<h3>Why Write a Rationale?</h3>

<ul>
  <li><strong>Improves understanding:</strong> A rationale clarifies the purpose and importance of a requirement, reducing ambiguity and promoting consistency.</li>
  <li><strong>Facilitates decision-making:</strong> It provides key information about the requirement's impact and trade-offs, aiding project decisions.</li>
  <li><strong>Provides context:</strong> Rationale offers essential background that ensures alignment with stakeholder needs and project constraints.</li>
</ul>

<h3>Examples</h3>

<ul>
  <li><strong>Example 1:</strong> A financial software requirement to encrypt all stored data. The rationale: protecting sensitive customer financial information from unauthorized access.</li>
  <li><strong>Example 2:</strong> A mobile app requirement to support offline mode. The rationale: ensuring seamless user experience even without internet connectivity.</li>
  <li><strong>Example 3:</strong> A website requirement to be optimized for accessibility. The rationale: making the site usable for users with disabilities.</li>
</ul>

<h3>What About Poor Rationales?</h3>

<p>Consider a requirement for a website to include a scrolling marquee on the homepage because it is "cool" or "trendy." While this might seem appealing, the rationale is weak because it can distract users, reduce readability, be inaccessible to some users, and misalign with project goals. A well-written rationale would explain these issues and why the feature should be reconsidered or rejected.</p>

Writing a rationale explains why a requirement exists, ensuring clarity, alignment, and testability. It improves understanding, supports decision-making, and provides essential context for stakeholders.

Ellygent Blog: requirements-rationale

The Importance of Writing Clear Requirements Rationale