Metrics for Software Maintenance

When development of a software product is complete and it is released to the market, it enters the maintenance phase of its life cycle. During this phase the defect arrivals by time interval and customer problem calls (which may or may not be defects) by time interval are the de facto metrics. However, the number of defect or problem arrivals is largely determined by the development process before the maintenance phase. Not much can be done to alter the quality of the product during this phase. Therefore, these two de facto metrics, although important, do not reflect the quality of software maintenance. What can be done during the maintenance phase is to fix the defects as soon as possible and with excellent fix quality. Such actions, although still not able to improve the defect rate of the product, can improve customer satisfaction to a large extent. The following metrics are therefore very important:

  • Fix backlog and backlog management index
  • Fix response time and fix responsiveness
  • Percent delinquent fixes
  • Fix quality

4.3.1 Fix Backlog and Backlog Management Index

Fix backlog is a workload statement for software maintenance. It is related to both the rate of defect arrivals and the rate at which fixes for reported problems become available. It is a simple count of reported problems that remain at the end of each month or each week. Using it in the format of a trend chart, this metric can provide meaningful information for managing the maintenance process. Another metric to manage the backlog of open , unresolved , problems is the backlog management index (BMI).


As a ratio of number of closed, or solved , problems to number of problem arrivals during the month, if BMI is larger than 100, it means the backlog is reduced. If BMI is less than 100, then the backlog increased. With enough data points, the techniques of control charting can be used to calculate the backlog management capability of the maintenance process. More investigation and analysis should be triggered when the value of BMI exceeds the control limits. Of course, the goal is always to strive for a BMI larger than 100. A BMI trend chart or control chart should be examined together with trend charts of defect arrivals, defects fixed (closed), and the number of problems in the backlog.

Figure 4.5 is a trend chart by month of the numbers of opened and closed problems of a software product, and a pseudo-control chart for the BMI. The latest release of the product was available to customers in the month for the first data points on the two charts. This explains the rise and fall of the problem arrivals and closures. The mean BMI was 102.9%, indicating that the capability of the fix process was functioning normally. All BMI values were within the upper (UCL) and lower (LCL) control limits ”the backlog management process was in control. ( Note: We call the BMI chart a pseudo-control chart because the BMI data are autocorrelated and therefore the assumption of independence for control charts is violated. Despite not being "real" control charts in statistical terms, however, we found pseudo-control charts such as the BMI chart quite useful in software quality management. In Chapter 5 we provide more discussions and examples.)

Figure 4.5. Opened Problems, Closed Problems, and Backlog Management Index by Month


A variation of the problem backlog index is the ratio of number of opened problems (problem backlog) to number of problem arrivals during the month. If the index is 1, that means the team maintains a backlog the same as the problem arrival rate. If the index is below 1, that means the team is fixing problems faster than the problem arrival rate. If the index is higher than 1, that means the team is losing ground in their problem-fixing capability relative to problem arrivals. Therefore, this variant index is also a statement of fix responsiveness.

4.3.2 Fix Response Time and Fix Responsiveness

For many software development organizations, guidelines are established on the time limit within which the fixes should be available for the reported defects. Usually the criteria are set in accordance with the severity of the problems. For the critical situations in which the customers' businesses are at risk due to defects in the software product, software developers or the software change teams work around the clock to fix the problems. For less severe defects for which circumventions are available, the required fix response time is more relaxed . The fix response time metric is usually calculated as follows for all problems as well as by severity level:

Mean time of all problems from open to closed

If there are data points with extreme values, medians should be used instead of mean. Such cases could occur for less severe problems for which customers may be satisfied with the circumvention and didn't demand a fix. Therefore, the problem may remain open for a long time in the tracking report.

In general, short fix response time leads to customer satisfaction. However, there is a subtle difference between fix responsiveness and short fix response time. From the customer's perspective, the use of averages may mask individual differences. The important elements of fix responsiveness are customer expectations, the agreed-to fix time, and the ability to meet one's commitment to the customer. For example, John takes his car to the dealer for servicing in the early morning and needs it back by noon. If the dealer promises noon but does not get the car ready until 2 o'clock, John will not be a satisfied customer. On the other hand, Julia does not need her mini van back until she gets off from work, around 6 P.M. As long as the dealer finishes servicing her van by then, Julia is a satisfied customer. If the dealer leaves a timely phone message on her answering machine at work saying that her van is ready to pick up, Julia will be even more satisfied. This type of fix responsiveness process is indeed being practiced by automobile dealers who focus on customer satisfaction.

In this writer's knowledge, the systems software development of Hewlett-Packard (HP) in California and IBM Rochester's systems software development have fix responsiveness processes similar to the process just illustrated by the automobile examples. In fact, IBM Rochester's practice originated from a benchmarking exchange with HP some years ago. The metric for IBM Rochester's fix responsiveness is operationalized as percentage of delivered fixes meeting committed dates to customers.

4.3.3 Percent Delinquent Fixes

The mean (or median) response time metric is a central tendency measure. A more sensitive metric is the percentage of delinquent fixes. For each fix, if the turnaround time greatly exceeds the required response time, then it is classified as delinquent:


This metric, however, is not a metric for real-time delinquent management because it is for closed problems only. Problems that are still open must be factored into the calculation for a real-time metric. Assuming the time unit is 1 week, we propose that the percent delinquent of problems in the active backlog be used. Active backlog refers to all opened problems for the week, which is the sum of the existing backlog at the beginning of the week and new problem arrivals during the week. In other words, it contains the total number of problems to be processed for the week ”the total workload. The number of delinquent problems is checked at the end of the week. Figure 4.6 shows the real-time delivery index diagrammatically.

Figure 4.6. Real-Time Delinquency Index


It is important to note that the metric of percent delinquent fixes is a cohort metric. Its denominator refers to a cohort of problems (problems closed in a given period of time, or problems to be processed in a given week). The cohort concept is important because if it is operationalized as a cross-sectional measure, then invalid metrics will result. For example, we have seen practices in which at the end of each week the number of problems in backlog (problems still to be fixed) and the number of delinquent open problems were counted, and the percent delinquent problems was calculated. This cross-sectional counting approach neglects problems that were processed and closed before the end of the week, and will create a high delinquent index when significant improvement (reduction in problems backlog) is made.

4.3.4 Fix Quality

Fix quality or the number of defective fixes is another important quality metric for the maintenance phase. From the customer's perspective, it is bad enough to encounter functional defects when running a business on the software. It is even worse if the fixes turn out to be defective. A fix is defective if it did not fix the reported problem, or if it fixed the original problem but injected a new defect. For mission-critical software, defective fixes are detrimental to customer satisfaction. The metric of percent defective fixes is simply the percentage of all fixes in a time interval (e.g., 1 month) that are defective.

A defective fix can be recorded in two ways: Record it in the month it was discovered or record it in the month the fix was delivered. The first is a customer measure, the second is a process measure. The difference between the two dates is the latent period of the defective fix. It is meaningful to keep track of the latency data and other information such as the number of customers who were affected by the defective fix. Usually the longer the latency, the more customers are affected because there is more time for customers to apply that defective fix to their software system.

There is an argument against using percentage for defective fixes. If the number of defects, and therefore the fixes, is large, then the small value of the percentage metric will show an optimistic picture, although the number of defective fixes could be quite large. This metric, therefore, should be a straight count of the number of defective fixes. The quality goal for the maintenance process, of course, is zero defective fixes without delinquency.

What Is Software Quality?

Software Development Process Models

Fundamentals of Measurement Theory

Software Quality Metrics Overview

Applying the Seven Basic Quality Tools in Software Development

Defect Removal Effectiveness

The Rayleigh Model

Exponential Distribution and Reliability Growth Models

Quality Management Models

In-Process Metrics for Software Testing

Complexity Metrics and Models

Metrics and Lessons Learned for Object-Oriented Projects

Availability Metrics

Measuring and Analyzing Customer Satisfaction

Conducting In-Process Quality Assessments

Conducting Software Project Assessments

Dos and Donts of Software Process Improvement

Using Function Point Metrics to Measure Software Process Improvements

Concluding Remarks

A Project Assessment Questionnaire

Metrics and Models in Software Quality Engineering
Metrics and Models in Software Quality Engineering (2nd Edition)
ISBN: 0201729156
EAN: 2147483647
Year: 2001
Pages: 176 © 2008-2020.
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