Software developers often want to freeze the requirements following some initial requirements work and then proceed with development, unencumbered with those pesky changes. This is the classic waterfall paradigm. It doesn’t work well in most situations. It’s far more realistic to define a requirements baseline and then manage changes to that baseline. This article, adapted from my book More About Software Requirements (Microsoft Press, 2006), defines the requirements baseline and describes when to create one.
Baseline Defined
The term
baseline comes from the domain of configuration management. The IEEE Standard Glossary of Software Engineering Terminology defines a baseline as:
A specification or product that has been formally reviewed and agreed on, that thereafter serves as the basis for further development, and that can be changed only through formal change control procedures.
A baseline is given a unique name so that the project participants can refer to it unambiguously. Good configuration management practices allow the team to reconstruct accurately any previous baseline and all its components.
A requirements baseline is a snapshot in time that represents the agreed-upon, reviewed, and approved set of requirements committed to a specific product release. That “release” could be a complete delivered product or any interim development increment of the product. When stakeholders “sign off” on requirements, what they’re really doing is agreeing and committing to a specific requirements baseline (whether they think of it in those terms or not).
Once the project team establishes a requirements baseline, the team should follow a pragmatic change control process to make good business and technical decisions about adding newly requested functionality and altering or deleting existing requirements. Change control is not about stifling change. It’s about providing decision makers with the information that will let them make timely and appropriate decisions to modify the planned functionality. That planned functionality is the baseline.
The Requirements Baseline
Whereas the scope definition distinguishes what’s in from what’s out, the requirements baseline explicitly identifies only those requirements that the project will implement. A baseline is not a tangible item but rather a defined list of items. One possible storage location is a software requirements specification (SRS) document. If that SRS contains only—and all—the requirements for a specific product release, the SRS constitutes the requirements baseline for the release. However, the SRS might include additional, lower-priority requirements that are intended for a later release. Conversely, a large project might need several software, hardware, and interface specifications to fully define the baseline’s components. The goal is to provide the project stakeholders with a clear understanding of exactly what is intended to go into the upcoming release.
Perhaps you’re storing your requirements in a requirements management tool, rather than in documents. In that case, you can define a baseline as a specific subset of the requirements stored in the database that are planned for a given release. Storing requirements in a tool allows you to maintain an aggregated set of both currently committed requirements and planned future requirements. Some commercial RM tools include a base lining function to distinguish those requirements (perhaps even down to the specific version of each requirement) that belong to a certain baseline.
Alternatively, you could define a requirement attribute in the tool to hold the release number or other baseline identifier. Moving a requirement from one baseline to another is then a simple matter of changing the value for that requirement attribute. The attribute approach will work when each requirement belongs to only a single baseline. However, you might well allocate the same requirement (or different versions of the same requirement) to several baselines if you’re concurrently developing multiple versions of your product, such as home and professional versions. Tool support is essential for such complex baseline management.
When following an incremental or iterative development life cycle, the baseline for each iteration will represent just a fraction of the overall system’s functionality. A small project my team once worked on took this approach. This project worked in three-week release cycles. For each cycle, the BA specified the requirements that were to be designed, coded, integrated, and verified during the next three weeks. Each requirements baseline was therefore quite small. The product grew incrementally toward full functionality as the developer periodically released useful versions to the users.
When to Baseline
Business analysts sometimes struggle with exactly when to define a requirements baseline. It’s an important decision because establishing the baseline has the following implications:
Formal change control begins. Change requests are made against an established baseline. The baseline therefore provides the point of reference for each proposed change. Make sure your change control process and players are in place before you define any project baselines.
Project managers determine the staffing levels and budgets needed. There are five dimensions to a software project that must be managed: features, quality, schedule, staff, and budget. Once the features and quality goals are defined in the baseline, the project manager adjusts the other three dimensions to accomplish the project’s objectives. It can work the other way, too. If staff, budget, and/or schedule are pre-established by external forces, the baseline composition is necessarily constrained to fit inside the project box bounded by those limits.
Project managers make schedule commitments. Prior to base lining, requirements are still volatile and uncertain, so estimates are similarly volatile and uncertain. Once a baseline is established, the contents of the release should be sufficiently well understood so that managers can make realistically achievable commitments. The managers still need to anticipate requirements growth by including sensible contingency buffers in their committed schedules.
Base lining requirements too early can push your change process into overdrive. In fact, receiving a storm of change requests after defining a baseline could be a clue that your requirements elicitation activities were incomplete and perhaps ineffective. On the other hand, waiting too long to establish a baseline could be a sign of analysis paralysis. Perhaps the BA is trying too hard to perfect the set of requirements before handing them to the development team.
Keep in mind that requirements development attempts to define a set of requirements that is good enough to let the team proceed with construction at an acceptable level of risk. Use the checklist in Table 1 to judge when you’re ready to define a requirements baseline as a solid foundation for continuing the development effort.
Table 1. Factors to Consider Before Defining a Requirements Baseline
| Business Rules |
Determine whether you’ve identified the business rules that affect the system and whether you’ve specified functionality to enforce or comply with those rules. |
| Change Control |
Make sure a practical change control process is in place for dealing with requirement changes and that the change control board is assembled and chartered. Ensure that the change control tool you plan to use is in place and configured and that the tool users have been trained. |
| Customer Perspective |
Check back with your key customer representatives to see whether their needs have changed since you last spoke. Have new business rules come into play? Have existing rules been modified? Have priorities changed? Have new customers with different needs been identified? |
| Interfaces |
See if functionality has been defined to handle all identified external interfaces to users, other software systems, hardware components, and communications services. |
| Model Validation |
Examine any analysis models with the user representatives, perhaps by walking through test cases, to see if a system based on those models would let the users perform their necessary activities. |
| Prototypes |
If you created any prototypes, did appropriate customers evaluate them? Did the BA use the knowledge gained to revise the SRS? |
| Alignment |
Check to see if the defined set of requirements would likely achieve the project’s business objectives. Look for alignment between the business requirements, user requirements, and functional requirements. |
| Reviews |
Have several downstream consumers of the requirements review them. These consumers include designers, programmers, testers, documentation and help writers, human factors specialists, and anyone else who will base their own work on the requirements. |
| Scope |
Confirm that all requirements being considered for the baseline lie within the project scope as it is currently defined. The scope might have changed since it was originally defined early in the project. |
| TBDs |
Scan the documents for TBDs (details yet to be determined) and other open requirements issues. The TBDs represent requirements development work remaining to be done. |
| Templates |
Make sure that each section of the SRS template has been populated. Alternatively, look for an indication that certain sections do not apply to this project. Common oversights are quality requirements, constraints, and assumptions. |
| User Classes |
See whether you’ve received input from appropriate representatives of all the user classes you’ve identified for the product. |
| Verifiability |
Determine how you would judge whether each requirement was properly implemented. User acceptance criteria are helpful for this. |
You’re never going to get perfect, complete requirements. The BA and project manager must judge whether the requirements are converging toward a product description that will satisfy some defined portion of customer needs and is achievable within the known project constraints. Establishing a baseline at that point establishes a mutual agreement and expectation among the project stakeholders regarding the product they’re going to have when they’re done. Without such an agreed-upon baseline, there’s a good chance someone will be surprised by the outcome of the project. Software surprises are rarely good news.
Author: Karl Wiegers, Process Imapct
Karl Wiegers is Principal Consultant at Process Impact, www.processimpact.com. His interests include requirements engineering, project management, peer reviews, and process improvement. His most recent book is Software Requirements, 3rd Edition (Microsoft Press, 2013), co-authored with Joy Beatty.