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  1. 1. RISK Research Methods Dent 313 1
  2. 2. Risk Risk is the probability of some untoward event Definition – The probability that people who are exposed to certain risk factors will subsequently develop the disease more often than similar unexposed people Risk factors – factors associated with an increased risk of becoming diseased 2
  3. 3. This lecture The lecture describes how estimates of risk are obtained by observing the relationship between exposure to possible risk and the subsequent development of the disease – Looking forwards – Looking backwards 3
  4. 4. Risk Factors  Physical environment factors – Toxin, infectious agents, gas, pollutants  Social environment factors – Emotional illness, stress, loss of family members, culture  Behavioral factors – Smoking, driving without seat belts, inactivity  Inherited factors – Diabetes, cholesterol, triglyceride 4
  5. 5. Exposure to risk factors The exposed person – Has come in contact with risk factor – Or has manifested the factor in question – Before becoming ill Duration of exposure – At a single point in time Example: nuclear bomb in Hiroshima – Over a period of time Example: smoking 5
  6. 6. Amount of exposure Relevant questions – Ever been exposed – Current dose – Largest dose taken – Total cumulative dose – Years of exposure – Years since first exposure 6
  7. 7. Measures of risk factor-disease relationship Exposure dose-disease relationship may not exist with all risk factors – Relationship: cumulative doses of sun exposure & non-melanoma skin cancer – No relationship: episodes of severe sunburn & melanoma Thus, correct measure has to be chosen to confirm the association between risk factor and disease 7
  8. 8. Choice of appropriate measures Based on – Clinical and biological effects – Pathophysiology of the disease – Previous epidemiological studies 8
  9. 9. Recognizing risk It is easy to recognize the association of acute disease and risk factors clinically – Examples: radiation, sunburn, acute poisoning It is more difficult to establish association between risk factors and chronic conditions clinically – WHY 9
  10. 10. Information about risk Because – Long latency period between exposure and disease – Frequent exposure to risk factors – Low incidence of disease – Small risk from exposure – Common disease – Multiple causes of a disease 10
  11. 11. Long latency period between exposure and disease Chronic diseases have long latency period between the exposure and the 1st manifestation of the disease It might be years later – E.g., Hypertension & heart disease The original exposure might be forgotten The link between the disease and the risk factors is not readily clear 11
  12. 12. Common exposure to risk factors Common risk factors – Smoking, cholesterol in Heart disease Comparing patterns of disease between – Those with the risk factors – Those without the risk factors (certain population subgroups) E.g., All Mormons (no smoking) E.g., Vegetarians (no fat diet) Comparisons through cross-sectional studies Investigating subgroups with low exposure to risk factors gives more information about the true risk-disease association 12
  13. 13. Low incidence of disease The incidence of diseases is very low (even with common diseases) – Lung cancer in heavy smokers is 2/1000 – Doctors might witness some rare disease once or few times in their practice It is difficult to draw a conclusion about infrequent events 13
  14. 14. Small risk Chronic disease caused by several risk factors acting together The risk of a single factor alone is very small If the risk is small, then large number of cases is needed to demonstrate the association of disease and risk factors – Example: coffee and heart diseases If the risk is high you can establish conclusion easily – Hepatitis B and hepatoma 14
  15. 15. Common disease If the disease is ordinary or commonly occurring and its risk factor is already known – There is no incentive to find new risk factors – Examples: heart disease, cancer, stroke If the disease is rare, careful investigation about risk factors are carried out 15
  16. 16. Multiple causes and effects There is no one-to-one relationship between a risk factor and a disease – E.g., Hypertension & CHD – Some people with HT develop CHD while others don’t – Some people without HT develop CHD Multiple risk factors for each particular disease Dental caries is a multifactorial disease – Bacterial – Carbohydrate – Host factor 16
  17. 17. Uses of risk Prediction of the occurrence of disease Search for cause Diagnosis Screening Prevention 17
  18. 18. Prediction of the occurrence of disease The quality of prediction depends on similarity of an individual patient with – A large number of patients – Who have past experience of the condition – With similar risk factors On an individual level, presence of a strong risk factor does not mean that the person is very likely to get the disease Prediction is expressed as a probability No better way than to use probability to guide clinical decision making at the individual level 18
  19. 19. Search for cause Search for risk factor is search for cause Causes – Immediate. E.g., virus – infection – Distant. E.g., maternal education – low birth wt A risk factor predicting disease is not necessarily a causal factor Marker: non-causal risk factor – Risk factor may mark the disease outcome indirectly – It is called marker because it marks the increase probability of the disease 19
  20. 20. Diagnosis The presence of a risk factor increase the probability that a disease is present – Therefore, knowledge of risk factor can be used in the diagnosis process The absence of risk factor helps to rule out a disease – Absence of high fluoride intake rules out fluorosis and strengthens other possibilities 20
  21. 21. Screening Knowledge of risk factors improves the efficiency of screening programs – By selecting subgroups at high risk – E.g., Risk of breast cancer is high among women with affected young women relatives 21
  22. 22. Prevention Removal of risk factor can prevent the disease regardless whether or not the mechanism of action of the risk factor in known – Stopping drinking of certain water (risk factor) prevents cholera infection in people – Stopping water with F > 1 ppm prevents fluorosis 22
  23. 23. Studies of risk Conducting an experiment helps to determine whether exposure to a potential risk develops disease People without disease divided into 2 groups – One subjected to risk factor – The other group is not – Otherwise the 2 groups are treated the same 23
  24. 24. When aren’t experiments possible Unethical to impose hazardous risk factors on healthy people for the purpose of a scientific research People hate to have their behavior modified by others for long period of time Experiments can be expensive to run Therefore, the choice goes in these situations towards observational studies 24
  25. 25. Observational studies Are clinical studies in which the researcher gathers data by simply observing events as they happened Have more potential for bias than experimental studies Most studies of risk factor Types – Cohort studies – Case-control studies 25
  26. 26. Cohort studies Cohort: a group of people with something in common in assembly who are observed for a period of time to see what happens to them Two condition to conduct cohort study  They do not have the disease at the time they are assembled  They should be observed for a meaningful period of time in the nature history of the disease in question  Enough time for the risk to be expressed  All member of the cohort should be observed over the full period of time 26
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  28. 28. Cohort study People assembled has not experienced the outcome but have equal susceptibility to develop the outcome People are then observed over a period of time Examine which people experience the outcome Other synonyms – Incidence studies – Longitudinal studies – Prospective studies 28
  29. 29. Cohort study Disease outcome YES Exposed to risk factor NO People at risk TIME YES Not exposed to risk factor No 29
  30. 30. Types of cohort studies Concurrent (prospective) Historical (retrospective) 30
  31. 31. Concurrent cohort The group of people (cohort) are assembled in the present and followed in the future The data are collected for the purpose of the study with full anticipation of what is needed – Bias can be avoided – Accuracy can be increased 31
  32. 32. Historical cohort studies Cases are assembled in the past and followed forward to the present The data are collected from available past records of patients Data may not be of sufficient quality for rigorous research Example: study cohort using dental records 32
  33. 33. Cohort studies PAST Cases assembled PRESENT FUTURE been followed-up HISTORICAL COHORT STUDY Cases assembled To be followed-up CONCURENT COHORT STUDY 33
  34. 34. Advantages of Cohort studies  The only way of establishing incidence directly  Can assess the relationship between exposure and many diseases  Best substitutes for true experimental studies when not possible  Follow the same logic as a clinical trial  Allow measurement of exposure to a risk factor  Avoid bias because the “unknown” but “expected” outcome develops after exposure to risk factor not vice versa 34
  35. 35. Disadvantages of cohort studies – Need large number of people at risk – The people must remain under the study for a long period of time – Can not be used for rare diseases – Expensive to run – Subjects are “free living” and not under control as in experimental studies – Expensive to keep track of them – Need resources employed for a long time – Usually limited to life-threatening diseases to justify the big budget 35
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