The Comparison of the Software Cost Estimating Methods Comparison Estimation Methods - RV

1. Expert Judgment Method The estimating steps using this method: Coordinator presents each expert with a specification and an estimation form. Coordinator calls a group meeting in which the experts discuss estimation issues with the coordinator and each other. Experts fill out forms anonymously Coordinator prepares and distributes a summary of the estimation on an iteration form. Coordinator calls a group meeting, specially focusing on having the experts discuss points where their estimates varied widely. Experts fill out forms, again anonymously, and steps 4 and 6 are iterated for as many rounds as appropriate. Comparison Estimation Methods - RV

The advantages of this method are: The experts can factor in differences between past project experience and requirements of the proposed project. The experts can factor in project impacts caused by new technologies, architectures, applications and languages involved in the future project and can also factor in exceptional personnel characteristics and interactions, etc. Comparison Estimation Methods - RV

The disadvantages include: This method can not be quantified. It is hard to document the factors used by the experts or expert-group. Expert may be biased, optimistic, or pessimistic, even though they have been decreased by the group consensus. The expert judgment method always complements the other cost estimating methods such as algorithmic method. Comparison Estimation Methods - RV

Comparison Estimation Methods - RV Estimating by Analogy The steps using estimating by analogy are: Characterising the proposed project. Selecting the most similar completed projects whose characteristics have been stored in the historical data base. Deriving the estimate for the proposed project from the most similar completed projects by analogy. Comparison Estimation Methods - RV

The main advantages of this method are: The estimates are based on actual project characteristic data. The estimator's past experience and knowledge can be used which is not easy to be quantified. The differences between the completed and the proposed project can be identified and impacts estimated. Comparison Estimation Methods - RV

However there are also some problems with this method, Using this method, we have to determine how best to describe projects. The choice of variables must be restricted to information that is available at the point that the prediction required. Possibilities include the type of application domain, the number of inputs, the number of distinct entities referenced, the number of screens and so forth. Comparison Estimation Methods - RV

Comparison Estimation Methods - RV Analogy problems Once we have characterized the project, we have to determine the similarity and how much confidence we can place in the analogies. Too few analogies might lead to maverick projects being used; too many might lead to the dilution of the effect of the closest analogies. Martin Shepperd etc. introduced the method of finding the analogies by measuring Euclidean distance in n-dimensional space where each dimension corresponds to a variable. Values are standardized so that each dimension contributes equal weight to the process of finding analogies. Generally speaking, two analogies are the most effective. Comparison Estimation Methods - RV

Comparison Estimation Methods - RV Analogy problems Finally, we have to derive an estimate for the new project by using known effort values from the analogous projects. Possibilities include means and weighted means which will give more influence to the closer analogies. Comparison Estimation Methods - RV

Comparison Estimation Methods - RV Top-Down Estimating It focuses on system-level activities such as integration, documentation, configuration management, etc., many of which may be ignored in other estimating methods and it will not miss the cost of system-level functions. It requires minimal project detail, and it is usually faster, easier to implement. Comparison Estimation Methods - RV

The disadvantages are: It often does not identify difficult low-level problems that are likely to escalate costs and sometime tends to overlook low-level components. It provides no detailed basis for justifying decisions or estimates. Comparison Estimation Methods - RV

Comparison Estimation Methods - RV Bottom-up Estimating The advantages: It permits the software group to handle an estimate in an almost traditional fashion and to handle estimate components for which the group has a feel. It is more stable because the estimation errors in the various components have a chance to balance out. Comparison Estimation Methods - RV

Comparison Estimation Methods - RV The disadvantages: It may overlook many of the system-level costs (integration, configuration management, quality assurance, etc.) associated with software development. It may be inaccurate because the necessary information may not be available in the early phase. It tends to be more time-consuming. It may not be feasible when either time and personnel are limited. Comparison Estimation Methods - RV

Comparison Estimation Methods - RV Algorithmic Method General advantages: It is able to generate repeatable estimations. It is easy to modify input data, refine and customise formulas. It is efficient and able to support a family of estimations or a sensitivity analysis. It is objectively calibrated to previous experience. Comparison Estimation Methods - RV

General disadvantages: It is unable to deal with exceptional conditions, such as exceptional personnel in any software cost estimating exercises, exceptional teamwork, and an exceptional match between skill-levels and tasks. Poor sizing inputs and inaccurate cost driver rating will result in inaccurate estimation. Some experience and factors can not be easily quantified. Comparison Estimation Methods - RV

Comparison Estimation Methods - RV Model Calibration The act of calibration standardises a model. Many models are developed for specific situations and are, by definition, calibrated to that situation. Such models usually are not useful outside of their particular environment. So, the act of calibration is needed to increase the accuracy of one of these general models by making it temporarily a specific model for whatever product it has been calibrated for. Calibration is in a sense customising a generic model. Items which can be calibrated in a model include: product types, operating environments, labor rates and factors, various relationships between functional cost items, and even the method of accounting used by a contractor. All general models should be standardised (i.e. calibrated), unless used by an experienced modeler with the appropriate education, skills and tools, and experience in the technology being modeled. Comparison Estimation Methods - RV

Comparison Estimation Methods - RV Calibration Calibration is the process of determining the deviation from a standard in order to compute the correction factors. For cost estimating models, the standard is considered historical actual costs. The calibration procedure is theoretically very simple. It is simply running the model with normal inputs (known parameters such as software lines of code) against items for which the actual cost are known. These estimates are then compared with the actual costs and the average deviation becomes a correction factor for the model. In essence, the calibration factor obtained is really good only for the type of inputs that were used in the calibration runs. For a general total model calibration, a wide range of components with actual costs need to be used. Better yet, numerous calibrations should be performed with different types of components in order to obtain a set of calibration factors for the various possible expected estimating situations. Comparison Estimation Methods - RV

Some recommendations: Do not depend on a single cost or schedule estimate. Use several estimating techniques or cost models, compare the results, and determine the reasons for any large variations. Document the assumptions made when making the estimates. Monitor the project to detect when assumptions that turn out to be wrong jeopardize the accuracy of the estimate. Improve software process: An effective software process can be used to increase accuracy in cost estimation in a number of ways. Maintaining a historical database Comparison Estimation Methods - RV