Copyright Chris Johnson, 2002.
Here is a printable version.

Safety-Critical Systems Development

Open Assessment 2002-2003

A. Introduction

Risk homeostasis is a controversial theory. It suggests that individuals have an implicit, preferred or target level of risk. One consequence of this theorem is that any safety improvements that reduce the risk exposure of an indiviudual will potentially lower the perceived threat below the target level. This creates the opportunity for people to alter their behaviour. In particular, they may trade performance objectives for a slightly increased level of risk. For example, car drivers that have access to advanced braking systems and other protection mechanisms may drive faster and brake later than drivers whose cars do not offer this higher degree of protection. Many people disagree with the basic ideas behind risk homeostasis theory. Does anyone really have an implicit target level of risk? Is anyone calculating enough to identify performance benefits that can be precisely traded off against safety improvements? Your task is to select an appropriate analytical technique to gather evidence about this theory. Gerald Wilde provides an on-line introduction to Risk Homeostasis. Be aware, however, that there are different perspectives on this issue and that you should also read more widely about the subject.

B. Your Task

Your task is to find evidence that will support or weaken the arguments made about risk homeostasis. You can use any method that you think is justified to support your arguments. The only caveats are that the evidence MUST be original and that at no point should you endanger anyone who may be connected with the project. The following paragraphs identify some of the techniques that can be used to provide evidence for and against risk homeostasis.

C. Methodologies for Examining Risk Homeostasis

There are many different ways in which you can provide or disprove assertions about risk homeostasis theory. These are summarised in the following paragraphs:

D. Transferable Skills

It is important to stress that this assessment will provide anumber of generic skills. Although we are focussing on evidence that might support or weaken arguments about risk homeostasis in general, the methods that you are using are identical to those that safety-critical organisations might use in order to assess the risks that are associated with particular products or designs. Statistical surveys or epidemiological studies can be used to identify benchmark figures for the performance of existing safety-critical systems. The existing frequency of injuries involving particular types of devices provides a target for development, any new systems must at least be as safe as previous applications. Experimental studies of risk taking behaviour provide means of assessing the actual performance of system designs prior to their full operational use. The same empirical methods that you might employ in this project can also be used on a commercial setting. The same criticisms can be raised about these methods as well. Epidemiological or experimental work can often be compromised by problems of under-reporting and gathering raw information about previous incidents involving particular devices. Similarly, operator behaviour in lab-based studies may have little relationship to their real-world performance.

E. Assessment Criteria and Submission Details

This exercise is degree assessed. It contributes 30% to the total marks associated with this course. The body of the report should not exceed fifteen A4 pages. The report must be printed out and must be submitted in a secure binder (i.e., one that will keep the pages together and in the correct order). It must include:

  1. A title page containing your student as well as your contact details (email address etc);
  2. A table of contents and appropriate page numbers;
  3. A section on the methodology that you used. This should begin with a statement of the hypothesis that you chose to prove or disprove. Please note that your project might look only at a specific aspect of risk homeostasis, such as the existinace of implicit risk targets rather than behaviour changes. This should include some consideration of alternative approaches and a considered justification of the reasons why you chose the method that you did.
  4. A results sections. This should describe the findings that you obtained. It should also discuss any problems that arose during the study that might make it difficult to interpret your findings.
  5. Conclusions. You must provide a clear statement about whether or not your study supports the ideas behind risk homeostasis. If your study indicates a mixed set of conclusions then you should state which aspects of the theory you are willing to support. You should also identify the limitations of the theory as it currently exists,
In addition to the fifteen pages associated with the body of the report, you may also include appendices. These should contain:
  1. the listing of any code used during the study together with suitable acknowledgements for the source of code that has been borrowed from other programmers;
  2. source data for any statistical study. The intention is that another analysts should be able to recreate your results in order to validate your findings. If the source data comes from the web or is available in another electronic form it is acceptible to simply provide a reliable URL or a disk.
It should be handed in at the start of the lecture on Wednesday 11th December 2002. Extensions will only be granted in exceptions circumstances and they should be requested prior to the deadline. Extensions for medical reasons should be reported as soon as possible and should be supported by forms from a medical practitioner. Extensions for equipment failures may be granted provided that you let me know as soon as they occur; so that I can make sure they get fixed as soon as possible. Please make sure that you keep back-up copies of all of your work towards this exercise.

The following marking scheme will be applied:

All solutions must be the work of the individual submitting the exercise. If any code or design ideas are borrowed from course notes, books or other students then those sources MUST be clearly acknowledged. All questions about this exercise should be addressed to Chris Johnson.

Back to the main index...