Question

A software manager is in charge of the development of a safety-critical software system that is designed to control a radiotherapy machine to treat patients suffering from cancer. This system is embedded in the machine and must run on a special-purpose processor with a fixed amount of memory (8 Mbytes). The machine communicates with a patient database system to obtain the details of the patient and, after treatment, automatically records the radiation dose delivered and other treatment details in the database. The COCOMO method is used to estimate the effort required to develop this system and an estimate of 26 person-months is computed. All cost driver multipliers were set to 1 when making this estimate. Explain why this estimate should be adjusted to take project, personnel, product and organisational factors into account. Suggest four factors that might have significant effects on the initial COCOMO estimate and propose possible values for these factors. Justify why you have included each factor.

Short answer

Adjust estimates for complexity, reliability, team skill, and tooling. These factors reflect real-world changes in required effort.

Step-by-step solution

Understanding COCOMO and Estimates

The Constructive Cost Model (COCOMO) is an algorithmic software cost estimation model that uses a basic equation using programmer effort (person-months). The initial estimate of 26 person-months means that without adjustment for project context, the effort calculated doesn't consider real-world variables that can significantly alter the needed resources.

Introduction of Cost Drivers

Cost drivers are factors used in COCOMO to adjust estimates based on specific project characteristics. These drivers fall into four categories: product, personnel, platform, and project factors, all of which can cause variations from initial estimates if not considered.

Identifying Four Key Factors

We need to adjust the estimate considering: 1. Product Complexity, 2. Required Software Reliability, 3. Experience and Capability of the Team, 4. Use of Software Tools. Each of these factors can significantly change the workloads due to the specifics of the project.

Factor Justification and Value Proposal - Product Complexity

Product Complexity: The radiotherapy software system is highly complex, possibly suggesting an increase in the multiplier. A propounded value could be High Complexity with a multiplier above 1, reflecting additional effort required for a sophisticated embedded system.

Factor Justification and Value Proposal - Required Software Reliability

Required Software Reliability: Safety-critical software mandates high reliability, implying a need for thorough testing and validation, significantly raising effort. Reliability might take a multiplier of greater than 1, indicating stringent quality assurance processes and additional work.

Factor Justification and Value Proposal - Experience and Capability of Team

Experience and Capability of Team: Adjust effort based on the expertise and familiarity of the team with similar systems. If team experience is average or low, the multiplier should be more than 1, whereas skilled teams with prior experience can apply a multiplier less than 1.

Factor Justification and Value Proposal - Use of Software Tools

Use of Software Tools: Advanced supportive tools can greatly enhance productivity, reducing effort needs. More sophisticated tools can offer a multiplier less than 1, assisting in automation or simplifying complex tasks during development and testing.

Key concepts

COCOMO Model

The Constructive Cost Model, or COCOMO, is a predictive tool used to estimate the cost required for software development projects. It provides estimates by calculating the amount of effort in terms of person-months needed to complete a project. This is achieved through a simple equation that factors in variables related to the complexity and scale of the project. However, the base estimate generated by COCOMO might not suffice because it doesn't automatically include critical project-specific attributes.

To enhance the accuracy of predictions, COCOMO incorporates cost drivers, which adjust the model according to various conditions specific to the project. These cost drivers are categorized into product, personnel, platform, and project factors, providing a realistic view of what the actual costs might be. For safety-critical systems, such as radiotherapy machine software, these adjustments are crucial to ensure comprehensive planning and resource allocation.

Safety-Critical Systems

Safety-critical systems are those that could be hazardous if they fail, like the control system for a radiotherapy machine. Ensuring reliability and correctness is paramount as these systems directly affect human health and safety. In a software context, such systems must be meticulously designed, verified, and tested to meet high standards of reliability and safety. For systems like these, components such as built-in redundancies, thorough validation processes, and rigorous testing protocols are essential to prevent failures.

The COCOMO model may require specific adjustments for safety-critical systems, accounting for the increased efforts in testing and validation. These processes often require more time and skill, impacting the cost estimate significantly, as reflected in increased effort multipliers.

Effort Adjustment Factors

Effort adjustment factors (EAF) are crucial in tailoring COCOMO estimates to fit the unique attributes of a project. They recognize that no two software projects are identical and adjust the base estimate to accommodate various conditions. The key factors usually considered include product complexity, required software reliability, team experience, and the adoption of software tools. Each of these can either increase or decrease the effort multiplier:

• Product Complexity: Sophisticated systems will have higher multipliers due to the additional resources needed for development.
• Software Reliability: Systems requiring high reliability, such as safety-critical software, will see an increase in effort due to the need for comprehensive testing.
• Team Experience: A well-experienced team might have a lower multiplier, optimizing productivity through their expertise.
• Use of Software Tools: Advanced tools can reduce the multiplier by automating complex tasks, enhancing productivity.

Adjusting these factors ensures that effort estimates align with the actual project demands.

Software Project Management

Software project management involves planning, executing, and overseeing software projects from conception to completion. This field is critical, especially for sophisticated projects like the development of safety-critical systems. Strong management ensures that the project doesn't deviate from its objectives, timelines, and budget. Project managers leverage models like COCOMO to estimate costs and allocate resources effectively. They use effort adjustment factors to tailor these estimates, aligning project executions with goals and constraints. Additionally, software project management emphasizes risk management, quality assurance, and stakeholder communication.

Planning for software tools, tracking team dynamics, and foreseeing potential project risks are all vital parts of effective software project management, particularly in fields where precision and reliability are non-negotiable.