The ISO 9126-1 software quality model identifies 6 main quality characteristics, namely:

1. Functionality
2. Reliability
3. Usability
4. Efficiency
5. Maintainability
6. Portability

These characteristics are broken down into sub characteristics, a high level table is shown below. It is at the sub characteristic level that measurement for SPI will occur. The main characteristics of the ISO9126-1 quality model, can be defined as follows:- 

1. Functionality : Suitability, Accuracy, Security, Interoperability, Compliance
Functionality is the essential purpose of any product or service. For certain items this is relatively easy to define, for example a ship’s anchor has the function of holding a ship at a given location. The more functions a product has, e.g. an ATM machine, then the more complicated it becomes to define it’s functionality. For software a list of functions can be specified, i.e. a sales order processing systems should be able to record customer information so that it can be used to reference a sales order. A sales order system should also provide the following functions:

• Record sales order product, price and quantity.
• Calculate total price.
• Calculate appropriate sales tax.
• Calculate date available to ship, based on inventory.
• Generate purchase orders when stock falls below a given threshold.

The list goes on and on but the main point to note is that functionality is expressed as a totality of essential functions that the software product provides. It is also important to note that the presence or absence of these functions in a software product can be verified as either existing or not, in that it is a Boolean (either a yes or no answer). The other software characteristics listed (i.e. usability) are only present to some degree, i.e. not a simple on or off. Many people get confused between overall process functionality (in which software plays a part) and software functionality. This is partly due to the fact that Data Flow Diagrams (DFDs) and other modeling tools can depict process functionality (as a set of data in\data out conversions) and software functionality. Consider a sales order process, that has both manual and software components. A function of the sales order process could be to record the sales order but we could implement a hard copy filing cabinet for the actual orders and only use software for calculating the price, tax and ship date. In this way the functionality of the software is limited to those calculation functions. SPI, or Software Process Improvement is different from overall Process Improvement or Process Re-engineering, ISO 9126-1 and other software quality models do not help measure overall Process costs\benefits but only the software component. The relationship between software functionality within an overall business process is outside the scope of ISO 9126 and it is only the software functionality, or essential purpose of the software component, that is of interest for ISO 9126. 

Following functionality, there are 5 other software attributes that characterize the usefulness of the software in a given environment. 
Each of the following characteristics can only be measured (and are assumed to exist) when the functionality of a given system is present. In this way, for example, a system can not possess usability characteristics if the system does not function correctly (the two just don’t go together).

2.Reliability : Maturity, Fault Tolerance, Recoverability, Compliance
Once a software system is functioning, as specified, and delivered the reliability characteristic defines the capability of the system to maintain its service provision under defined conditions for defined periods of time. One aspect of this characteristic is fault tolerance that is the ability of a system to withstand component failure. For example if the network goes down for 20 seconds then comes back the system should be able to recover and continue functioning. 

3.Usability : Understandability, Learn-ability, Operability, Attractiveness, Compliance
Usability only exists with regard to functionality and refers to the ease of use for a given function. For example a function of an ATM machine is to dispense cash as requested. Placing common amounts on the screen for selection, i.e. $20.00, $40.00, $100.00 etc, does not impact the function of the ATM but addresses the Usability of the function. The ability to learn how to use a system (learnability) is also a major subcharacteristic of usability. 

4.Efficiency : Time behaviour, Resource behaviour, Compliance
This characteristic is concerned with the system resources used when providing the required functionality. The amount of disk space, memory, network etc. provides a good indication of this characteristic. As with a number of these characteristics, there are overlaps. For example the usability of a system is influenced by the system’s Performance, in that if a system takes 3 hours to respond the system would not be easy to use although the essential issue is a performance or efficiency characteristic. 

5.Maintainability : Analyzability, Change-ability, Stability, Testability, Compliance
The ability to identify and fix a fault within a software component is what the maintainability characteristic addresses. In other software quality models this characteristic is referenced as supportability. Maintainability is impacted by code readability or complexity as well as modularization. Anything that helps with identifying the cause of a fault and then fixing the fault is the concern of maintainability. Also the ability to verify (or test) a system, i.e. testability, is one of the subcharacteristics of maintainability. 

6.Portability : Adaptability, Install-ability, Co-existence, Repiace-ability, Compliance
This characteristic refers to how well the software can adopt to changes in its environment or with its requirements. The subcharacteristics of this characteristic include adaptability. Object oriented design and implementation practices can contribute to the extent to which this characteristic is present in a given system.