Project Estimation with Use Case Points

Introduction

Object-oriented applications frequently rely on Use Cases to describe the business processes of a software application. Since use cases consist of the strategic goals and scenarios that provide value to a business domain, they can also provide insight into an application’s complexity. Deriving a reliable estimate of the size and effort an application needs, is possible by examining the actors and scenarios of a use case. Use Case Points is a project estimation method that employs a project’s use cases to produce an accurate estimate of a project’s size and effort.

Use Case Points

Use case modeling is an accepted and widespread technique to capture the business processes and requirements of a software application. Since they provide the functional scope of the application, analyzing their contents provides valuable insight into the effort and size needed to design and implement the application. In general, applications with large, complicated use cases take more effort to design and implement than small applications with less complicated use cases. Moreover, the time to complete the application is affected by:

• The number of steps to complete the use case.
• The number and complexity of the actors.
• The technical requirements of the use case such as concurrency, security and performance.
• Various environmental factors such as the development teams’ experience and knowledge.

Use Case Points (UCP) is an estimation method that provides the ability to estimate an application’s size and effort from its use cases. Based on work by Gustav Karner in 1993, UCP analyzes the use case actors, scenarios and various technical and environmental factors and abstracts them into an equation.

The equation is composed of four variables:

1. Technical Complexity Factor (TCF).
2. Environment Complexity Factor (ECF).
3. Unadjusted Use Case Points (UUCP).
4. Productivity Factor (PF).

Each variable is defined and computed separately, using perceived values and various constants. The complete equation is:

UCP = TCP * ECF * UUCP * PF

The necessary steps to generate the estimate based on the UCP method are:

1. Determine and compute the Technical Factors.
2. Determine and compute the Environmental Factors.
3. Compute the Unadjusted Use Case Points.
4. Determine the Productivity Factor.
5. Compute the product of the variables.

Technical Complexity Factors

Thirteen standard technical factors exist to estimate the impact on productivity that various technical issues have on an application. Each factor is weighted according to its relative impact. A weight of 0 indicates the factor is irrelevant and the value 5 means that the factor has the most impact.

Figure 1: Technical Factors.

For each project, the technical factors are evaluated by the development team and assigned a value from 0 to 5 according to their perceived complexity – multithreaded apps. require more skill and time than single threaded applications, for example, as do reusable apps. A perceived complexity of 0 means the technical factor is irrelevant for this project; 3 is average; 5 means it has strong influence.

Each factor’s weight is multiplied by its perceived complexity to produce its calculated factor. The calculated factors are summed to produce the Total Factor.

So, using sample perceived complexity values, the Technical Total Factor might be computed as follows:

Figure 2: Calculating the Technical Total Factor.

In Figure 2, the Total Factor is 47 derived by summing all the calculated factors. To produce the final TCF, two constants are computed with the Total Factor. The complete formula to compute the TCF is as follows:

TCF = 0.6 + (.01*Total Factor). For Figure 1, the TCF = 1.07

Environmental Complexity Factors

Environmental Complexity estimates the impact on productivity that various environmental factors have on an application. Each environmental factor is evaluated and weighted according to its perceived impact and assigned a value between 0 and 5. A rating of 0 means the environmental factor is irrelevant for this project; 3 is average; 5 means it has strong influence.

Figure 3: Example Environmental Factors.

Each factor’s weight is multiplied by its perceived complexity to produce its calculated factor. The calculated factors are summed to produce the Total Factor.

Using sample values for perceived impact, the Environmental Total Factor might be computed as:

Figure 4: Calculating the Environmental Total Factor.

In Figure 4, the Total Factor is 26 derived by summing all the calculated factors. To produce the final ECF, two constants are computed with the Total Factor. The complete formula to compute the ECF is as follows:

ECF = 1.4 + (-0.03*Total Factor). For Figure 4, the ECF = 0.62

Unadjusted Use Case Points (UUCP)

Unadjusted Use Case Points are computed based on two computations:

1. The Unadjusted Use Case Weight (UUCW) based on the total number of activities (or steps) contained in all the use case Scenarios.
2. The Unadjusted Actor Weight (UAW) based on the combined complexity of all the use cases Actors.

UUCW

Individual use cases are categorized as Simple, Average or Complex, and weighted depending on the number of steps they contain - including alternative flows.

Figure 5: Use Case Categories.

The UUCW is computed by counting the number of use cases in each category, multiplying each category of use case with its weight and adding the products.

Figure 6: Computing UUCW.

UAW

In a similar manner, the Actors are classified as Simple, Average or Complex based on their interactions.

Figure 7: Actor Classifications.

The UAW is calculated by counting the number of actors in each category, multiplying each total by its specified weighting factor, and then adding the products.

Figure 8: Computing UAW.

Finally, the UUCP is computed by adding the UUCW and the UAW. For the sample data used in the figures, the UUCP = 220 + 44 = 264.

Productivity Factor

The Productivity Factor (PF) is a ratio of the number of man hours per use case point based on past projects. If no historical data has been collected, a figure between 15 and 30 is suggested by industry experts. A typical value is 20.

Final Calculation

The Use Case Points is determined by multiplying all the variables:

UCP = TCP * ECF * UUCP * PF

For the sample values used in this article:

UCP = 1.07 * 0.62 * 264 * 20 = 3502.752 or 3503 hours.

Dividing the UCP by 40 hours (for one man work week) = 88 man-weeks. Therefore, for the sample values in this article, it would take one developer 88 weeks (or about 22 months) to complete the application.

Caveats

The Use Case Points estimate tends to be high when compared to human experts. This might be a good thing since many software projects are late, but, the estimate may still be too high. In order to produce accurate results, the variables in the equation need to be adjusted and tweaked – especially in the beginning.

• The number of steps in a scenario affects the estimate. A large number of steps in a use case scenario will bias the result towards complexity and increase the Use Case Points. A small number of steps will bias it towards simplicity. Sometimes, groups of steps can be reduced to a fewer number without sacrificing the business process. Strive for a uniform level of detail but don’t force a use case to conform to the estimation method.
• Including and extending use cases increases the complexity. Count these as a single use case.
• The number of actors in a use case also affects the estimate. If possible, generalize the actors into a single superactor. This reduces the complexity without affecting the use case. On the other hand, don’t force a generalization where none exists.
• The values for the Technical and Environmental Factors need to be adjusted over time as actual data is obtained. The more projects that employ Use Case Points for their estimations will yield more accurate values for the perceived values.
• Compare the Use Case Point estimate with a human expert’s estimate. Where there is disagreement, err on the side of the human expert and adjust the Use Case Point factors accordingly.
• The Productivity Factor can only be obtained over time. Track the time spent designing and implementing the use cases and adjust the Productivity Factor accordingly.

Conclusion

Use Case Points have the potential to produce reliable results because its estimates are produced from the actual business processes – the use cases - of a software application. Additionally, in many traditional estimation methods, influential technical and environmental factors are often not adequately given enough consideration. Use Case Points includes and abstracts these subjective factors into an equation. When tweaked, over time, Use Case Points can provide estimates that are very reliable.

Additional Links

1. Project Estimation using Use Case Metrics.
2. Test Effort Estimation Using Use Case Points.
3. Use Case Points Method.
4. How to Prepare Quotation Using Use Case Points.