Chapter 19: Quality Models andMeasurements • Types of Quality Assessment Models • Data Requirements and Measurement • Comparing Quality Assessment Models • Measurement and Model Selection
Introduction • Analytical models provide quantitative assessment of selected quality characteristics • Applied over time, provide accurate prediction of future quality • Purpose of measurement and analysis is to • make corrective actions =>improvement • provide timely feedback/assessment • identify problematic areas • prediction, anticipating/planning for scheduling and resource allocation
Models for Quality Assessment • Direct indicators of quality • defect measurements - defect density for correctness • probability of failure-free operation for reliability • measured at end of software development • Indirect indicators of quality • product internal attributes (e.g. KLOC, McCabe’s) • interaction between product and user • development process • general characteristics of product (e.g. telecom) • may be available early enough to make predictions
Models for Quality Assessment
Generalized Models for Quality Assessment • Require little or no project-specific data • Three categories • Overall model – provides a single estimate of overall product quality • Segmented model – provides different quality estimates for different industrial segments • Dynamic model – provides quality trend or distribution over time or development process
Overall Models • Most general subtype of generalized quality models • Provide a rough estimate of product quality, e.g. defect density = total defects / product size • Lump all products together – abstraction of commonly observed facts about quality generally true over all kinds of application domains, e.g. • 80:20 rule which states 80% of defects are concentrated in 20% of product modules/components • linkages between software defect, risk, process maturity to quality
Segmented Models • Abstraction of commonly observed facts about quality over product market segments, e.g. • reliability levels (measured by failure rate) • safety-critical SW – medical devices and nuclear reactors • commercial SW – telecommunications and business • auxiliary SW – games and low-cost PC SW
Dynamic Models • Provide information about quality over time or development phases, e.g. • defect distribution profile over dev. phases • Putnam model – effort and defect profiles over time • reliability growth during product testing • Can be combined with segmented models to give us segmented dynamic models
Product-Specific Models • Provide more precise quality assessments using product-specific data • Three categories • Semi-customized models – extrapolate product history to predict quality for the current project (Table 2) • Observation-based models – estimate quality based on observations from the current project • Measurement-driven predictive models – establish predictive relations between various early measurements and product quality
Semi-Customized Models • Use general characteristics and historical information about product, process, or envt • Provide quality extrapolations • Examples: • Defect removal models (DRMs) provide defect distribution profile over development phases based on previous releases of the same product • Combine DRM with orthogonal defect classification (ODC) model - profiles defects by individual phases in which they where injected, discovered, and by categories => identify high-defect areas
Observation-based Models • Relate observations of the software system behavior to information about related activities for more precise quality assessments, e.g. • SRGMs – estimate parameters based on observation data • Usually use data from current project
Measurement-driven predictive models • Establish predictive relations between quality and other measurements from historical data • Provide early predictions of quality • Identify problems early for timely actions • Use statistical analysis techniques / learning algorithms • Examples: • Relationships between defect fixes and design and code measurements • high-defect modules of legacy products associated with numerous changes and high data complexity • high-defect modules of new products associated with complex design and control structures
Identify High-risk areas in Development • Relationship between defect fixes and various design and code measurements • High-defect modules of legacy products associated with numerous changes and high data complexity • High-defect modules of new projects associated with complex design and control structures
Model Comparison and Interconnections • Comparisons based on • usefulness of modeling results, how accurate quality estimates are, and applicability of models to different environments • Model inter-connections examined in two opposite directions • Customization required of generalized quality models to create product-specific models • Generalization of product specific models when enough empirical evidence from different products or projects is accumulated
Comparisons • Usefulness can be weighted against cost(such as collecting data) • Generalized models more widely applicable and less expensive to use (do not require product-specific measurements) • Generalized models more useful in product planning stage and early development phases – when product-specific data unavailable, except when historical data exists in which case semi-customized models are better
More Comparisons • Observation-based and Measurement-based predictive models better manage QA activities and later development and maintenance activities as more measurement data collected
More Comparisons • Counterparts in generalized models to product-specific models and vice versa • Generalized models can be customized into product-specific ones • Product-specific models can be generalized • Depends on kind of measurement data collected and analysis results available
Data Requirements and Measurement • Different models have different data requirements (direct and/or indirect) • Generalized models • based on industrial averages and general profiles for all products or product segment. • No data from current project needed directly • But measurement taken at current project can be accumulated into empirical base to calibrate models for future applications
Data Requirements and Measurement for Product-Specific Models • Measurement-driven models • need direct quality measurements and indirect quality measurements (process, product and people) • need early measurements from historical / current releases • Semi-customized models • indirect environmental measurements to characterize current project • extrapolate quality estimates from previous releases • use course-grain activity measures
Data Requirements and Measurement for Product-Specific Models • Observation-based models • direct quality measurements • environmental characteristics assumed
Data Requirements and Measurement (Table 19.5)
Models Supported by Kinds of Data • Direct and indirect quality measurements from industry form empirical basis for generalized models • Direct quality measurements used in all product-specific models • product-specific extrapolations in semi-customized models • development activities in observation-based models • predicted by early measurements in measurement-driven models
Models Supported by Kinds of Data • Environmental measurements mainly used in semi-customized models • characterize current product to make extrapolations • Product internal measurements used in measurement-driven predictive models • early assessment of product quality • identify problematic areas • Activity measurements used by various models • course-grained used in semi-customized models, e.g. defect data grouped by phase. • fine-grained used in observation-based models • Summarized in Figure 19.3
Selecting Measurements and Models • Use a goal-oriented approach (GQM) • Set specific quality goals (e.g. high reliability) • Choose specific quality assessment models that can answer our concerns (e.g. SRGMs) • Choose appropriate measurements (e.g. failure and test execution time measurements) • Examples A - C in text.
