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 PRACTICE PROBLEMS, (Probability trees, etc.)  
 PRACTICE PROBLEMS, (Chronic risk calculations)


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  825 / 8927



Offered jointly through the School of Graduate Studies, and the School of Occupational and Public Health, Ryerson University

      Tim Sly, MSc, PhD, DPHI, CPHI(C)

      Room POD-253(A) Tel:416- 979-5000 x 7556 Fax: 416-979-5377

      E-mail:     WEB:

      Office hours will be posted on the PROTEUS front page and on the office door.


The measurement of risk is an important component of environmental epidemiology and a wide range of decision-making throughout environmental and health sciences. It is also central to modern approaches in resource allocation, policy-making, facility-siting, and evaluation. 

The course is in three parts.  The first is an introduction to probabilistic risk assessment (PRA). We will apply the principles from probability theory to predict the occurrence and consequences of rare events that would be catastrophic should they happen.  The second part will deal with the calculation and assessment of chronic risks, in other words, the determination of the risk of persons in specified scenarios developing a fatal cancer (or other non-carcinogenic fatal outcome) over a lifetime due to small exposure to carcinogenic or toxic substances. For this we use the US-EPA four-step model.  Interpretation and communication of these calculations are an important component.  

The third part addresses the perception of risk, leading to more effective communication of risk, and involves a range of behavioural sciences to identify the processes and the complex set of criteria by which people see and identify risks to health and the environment. Current Several videotaped segments and case studies will be used to illustrate these criteria. We will attempt to include a useful session on dealing with the media and interviews.

         OBJECTIVES: At the end of the course, the learner should be able to (1) calculate probabilities of events using Venn diagrams, formulæ, contingency tables, and probabilistic trees,

(2) calculate interpret and communicate any increased risk of mortality attributable to lifetime exposure to substances in the environment or workplace, using standard techniques of risk assessment,

(3) discuss the current theories of risk perception, decision-making under uncertainty, and the techniques for effective risk communication whether at an individual, or group level, or through media.


        TOPICS TO BE COVERED   [Extensive use will be made of examples and case studies]

INTRO: Origins and evolution of risk assessment, risk terminology and definitions


Dealing with uncertainty:   Probability theory applied to risk and uncertainty.  Formulae for basic operations ( + - x ÷ ), conditional probabilities, independence, mutual exclusivity   


Use of Venn diagrams, contingency tables, and trees to display and solve simple problems in probability. Bayes theorem application; stochastic v deterministic methods; clusters: identification and determination,  interpretation and framing


The R/A process in chronic risk settings - hazard identification, exposure assessment, toxicity assessment, risk characterization, using the 4 stage EPA model


Risk perception - the phenomenon - understanding it, and its significance.  


Risk communication - what it is and how to improve effective risk communication


Risk comparisons - the effective and the troublesome. Dealing with the media - tips and techniques.



A mid-term test based on the first part of the course (30%) approx week 5

A mid-term test based on the second part of the course (30%) approx week 8 or 9

A comprehensive final examination OR assignment  40%.   [To be discussed and decided]


No single textbook encompasses the range of material introduced during this course. A package of notes, calculation guide, problem set, tables, and readings has been prepared and will be available during the first week of class from ALICOS printing (Church/Gerrard Street) Ask for the ENH825/ES8927 course notes.

Additional handouts and other materials may be distributed during the term, and further materials and pages will be added to this site..






Students must submit assignments on time and write all tests and exams as scheduled


Assignments submitted for grading will be handed back within two weeks except for the final exam.


There will be no penalty for work missed for a justifiable reason. Students need to inform the instructor of any situation that arises during the semester that may have an adverse affect on their academic performance, and request any necessary considerations according to the policies and well in advance. Failure to do so will jeopardize any academic appeals.


Except in cases of accommodations for disabilities, where documentation is handled directly by the Access Centre, students must fill out an Academic Consideration form and submit it to their own program office


In addition, the following procedures must be followed as well:

o          Medical certificates – If a student is going to miss a deadline for an assignment, a test or an examination because of illness, he/she must submit a medical certificate (see for the certificate) to their program office within 3 working days of the missed assignment deadline, test or examination. The program office will notify the instructor that the documents have been received. It is the student’s responsibility to make arrangements with instructor for a make up exam.

o          Religious observance – While it is strongly encouraged that students make requests within the first two weeks of class, requests for accommodation of specific religious or spiritual observance must be presented to their program office no later than two weeks prior to the conflict in question (in the case of final examinations within two weeks of the release of the examination schedule). The student must submit a Request for Accommodation form ( to their program office. The office will notify the instructor when they have received the request form.

o          Other requests for Academic Consideration which are not related to medical or religious observation must be submitted in writing together with the Academic Consideration form to the student’s program office. The letter must clearly state the reasons for the request and describe the events or circumstances that seriously impair the student’s ability to meet their academic obligations, and that were beyond the student’s control. When possible, supporting documentation must be attached to the letter. The office will notify the instructor when they have received the request.

o         Students with disabilities - In order to facilitate the academic success and access of students with disabilities, these students should register with the Access Centre  Before the first graded work is due, students should also inform their instructor through an “Accommodation Form for Professors” that they are registered with the Access Centre and what accommodations are required.

o          Regrading or recalculation – These requests must be made to the instructor within 10 working days of the return of the graded assignment to the class.  These are not grounds for appeal, but are matters for discussion between the student and the instructor.

·        Submission of the Academic Consideration form and all supporting documentation to your program office does not relieve you of the responsibility to NOTIFY YOUR INSTRUCTOR of the problem as soon as it arises, and to contact with the instructor again after the documents have been submitted in order to make the appropriate arrangements.

·        If you do not have a justifiable reason for an absence and/or have not followed the procedure described above, you will not be given credit or marks for the work missed during that absence. 


For more detailed information on these issues, please refer to Senate Policy 134 at (Undergraduate Academic Consideration and Appeals) and Senate Policy 150 (Accommodation of Student Religious Observance Obligations). Both can be found at