Software Measurement: A Necessary Scientific Basis By Norman Fenton Presented by Siv Hilde Houmb Friday 1 November

Outline What is measurements? How to measure Direct and indirect measurement Representation of measurement Use of measurement Software measurement (different models)

What is measurements? Measurement is The process by which numbers of symbols are assigned to attributes of entities in the real world in such a way as to describe them according to clearly defined rules. Entity: Object (person, journey, software specification) Attribute: Feature (height, cost, the length of a specification)

How do we measure? What do we mean by assigning values? Describe the “world” Need roles on how to evaluate what is the longest, biggest etc. Models of the entities being measured Model must be specific in terms of describing phenomena we are studying

Direct and indirect measurement Direct measurement Measurement which does not depend on the measurement of any other attribute. Indirect measurement Measurement which involves the measurement of one or more other attributes.

Uses of measurement Assessment Collect data and assess (bedømmelse) Prediction Model Need to define procedure for Determine model parameters Interpret results Prediction system Model and procedure for determine model parameters and interpreting the result.

Measurement activity must have clear objectivities Assessment or prediction Entity and attribute Goal/Question/Metric GQM (Basili and Rombach)

Representation of measurement Empirical Relation Systems Empirical relation system (C,R), where C is the set of entities and R is the set of empirical relationship. Representation Condition Mathematically representation of an expression, for example taller than (what does that really mean (>) ) Numerical relation system (N,P), where entities in C are mapped to numbers in N and empirical relations in R are mapped to numerical representation in P.

Representation of measurement Scale types and Meaningfulness Measurement scale (inches, centimetres, scale for criticality in risk assessment etc.) Not only just a number, but the number relates to a scale

Measuring Software Complexity Comprehensibility Correctness Maintainability Reliability Testability Ease of implementation

Framework for Software Measurement Three classes of entities of interest Processes Products Resources Internal attributes Those which can be measured purely in terms of the product, process or resource itself. (eks. Length of product) External attributes Those which can only be measured with respect to how the product, process or resource relate to other entities in its environment. (eks. Reliability)

Software metrics Cost modelling Predicting the attribute of effort or time required for the process of development Eks. Size number of delivered source statements or function points

Software quality models Quality measured as Factors (external product attributes) Criteria (internal product/process attribute) Metrics (measurement of internal attributes)

Reliability models Measures external product attributes Predictive Reliability modelling is concerned with predicting reliability of software on the basis observing times between failures durig operation or testing.

Reliability models cont. Internal attributes of processes are used to predict an external product attribute. Prediction system Probability distribution model together with a statistically inference procedure for determining the model parameters, and a prediction procedure for combining the model and the parameters estimates to make a statement about future reliability.

Halstead`s software science Measure Length Vocabulary Volume Measure in terms of numbers of operators and operands. Does not mean program complexity!

Validating software measures Demonstrating empirically that the representation condition is satisfied for the attribute being measured Needs prediction system and experimental hypothesis

Summary We need to measure internal attributes to support the measurement of external attributes (eks. Reliability) The representation theory of measurements asserts that measurements is the process of assigning numbers or symbols to attributes of entities in such a way that all empirical relations are preserved

Summary cont. The entities of interest in software can be classified as processes, products and resources. In many cases we cannot measure directly external attributes -> must measure indirectly through internal attributes