Epidemiology

1. Epidemiology A branch of science that investigates the frequency and distribution of diseases in a defined population in an attempt to determine their causes, to discover ways to alleviate them, and to prevent their reoccurrences

2. Features of epidemiologic studies • Observe people in their natural setting • Over a specific period of time • At one point in time • Retrospectively • The objective is to describe specific traits that may be present among members of a population Evidence-based Chiropractic

3. Risk factorsa.k.a., predictor variables • Outcomes of groups of subjects are compared • One group was exposed and the other was not exposed to some risk factor • Did the exposed group develop a higher incidence of the disease • Valuable type of study because it is unethical for researchers to ask patients to do or take something known to be harmful Evidence-based Chiropractic

4. Epidemiologic studies considered observational • Data are collected via surveys, review of medical records, etc. • Data analyzed using statistical tests designed to identify patterns and correlations • e.g., people who lift heavy at work are more likely to develop low back pain Evidence-based Chiropractic

5. The epidemiologic approach • Its focus is on prevention rather than treatment • It deals with populations rather than individual patients • The approach is to identify subgroups that are at high-risk of developing a disease and then find out what factors caused persons in the subgroups to be at high-risk Evidence-based Chiropractic

6. The epidemiologic approach (cont.) • Preventive measures can be developed to minimize risk factors in persons found to be at high-risk • e.g., teaching construction workers to lift properly to prevent low back injuries • The effectiveness of the preventive measures can be monitored using epidemiologic methods Evidence-based Chiropractic

7. Epidemiologic studies • Preferred to RCTs when questions are about diagnosis, prognosis or causation • Related to ethical concerns and feasibility • Example studies • The link between driving heavy equipment and the incidence of lower back pain • Persons who consume enough calcium and vitamin D are less likely to develop osteoporosis Evidence-based Chiropractic

8. Epidemiologic studies (cont.) • Two groups are selected • One group is exposed to some agent or event, while the other is not • Any difference in the rate of disease between the groups may be because of the exposure • Statistical tests are used to determine the probability that other persons exposed to the same risk factors would develop the disease Evidence-based Chiropractic

9. More potential for bias in epidemiologic studies • Subjects are not randomly assigned to be in exposure versus no-exposure groups • Differences may exist in the baseline risk of disease between the groups • Random selection helps reduce the influence of many biases • Randomly selected samples are much more representative of the population Evidence-based Chiropractic

10. Evidence-based Chiropractic

11. Reliability of data sources • The validity of epidemiologic studies depends to a large extent on the reliability of the data that is collected • Often collected directly from people or by relying on others • e.g., mailed surveys, hospital emergency room personnel, physicians, etc. • These are all potentially unreliable sources Evidence-based Chiropractic

12. Reliability of data sources (cont.) • Questionnaires need to be properly constructed to ensure accurate information • No “double-barreled” questions • No leading questions • Respondents should be competent to answer the questions • The next slide is an example of a biased questionnaire from the Canadian Stroke Consortium – SPONTADS Study Evidence-based Chiropractic

13. There are many other daily activities that have been reported to be associated with cervical artery dissection. Why weren’t they listed as possible choices? Evidence-based Chiropractic

14. Canadian Stroke Consortium results were biased • Reported that 28% of the cervical artery dissections in their group were the result of manipulation • This is significantly higher than what other authors have reported • Approximately 6% between 1994 and 2003 • Cervical trauma/manipulation as the only choice called for a highly biased response Evidence-based Chiropractic

16. Face to-face interviews • The potential for bias due to leading questions is even higher • Interviewer facial expressions, body language, and vocal inflections can influence responses Evidence-based Chiropractic

17. Respondents should be competent to answer questions • Includes children, persons with dementia, etc. • Also automobile crash victims • They are typically quite upset and may be confused • Mild traumatic brain injury is fairly common following car crashes which may result in confusion Evidence-based Chiropractic

18. The best measures for epidemiologic research • Are as free from error as possible • Inexpensive, readily available, easy to use, etc. • Ethically acceptable • The resulting information is important • Change in the predictor variable is linked to change in the outcome variable Evidence-based Chiropractic

19. Common designs used in epidemiologic research • Cross-sectional studies • Case-control studies • Cohort studies • Which one to use depends on • The frequency of the disease or condition • The availability of human and economic resources Evidence-based Chiropractic

20. Cross-sectional studiesa.k.a., prevalence studies • Assess the health status and exposure levels of persons in a population at one point in time • Cases must actively manifest the disease to be included • Cases with developing conditions that have not yet been diagnosed are not counted Evidence-based Chiropractic

21. Cross-sectional study (cont.) • Purpose • To determine if there is an association between a suspected causal factor and a condition • They are useful to discover associations, but incapable of determining if one factor caused the other • Case-control or cohort studies are often used to verify their results Evidence-based Chiropractic

22. Cross-sectional study (cont.) • They are attractive to researchers because they are relatively quick and easy to carry out; and are inexpensive • However, it may take a long time to gather all of the required information • Consequently, they are often the initial research tools used to investigate exposures to risk factors and their relationships to disease Evidence-based Chiropractic

23. Cross-sectional study design At one point in time Exposure levels Health status Evidence-based Chiropractic

24. Case-control studies • A study that starts off by identifying two groups of subjects • One group has a disease or condition (cases) • The other group is free from the disease (controls) • Prior exposures of the cases are compared with those of the controls to see if the exposures influenced the odds of developing the disease Evidence-based Chiropractic

25. Subjects in case-control studies • Controls should be as similar to cases as is possible • Cases and controls are normally matched so that they are as alike as possible • Regarding variables such as age, gender, weight, occupation, etc. • Except for the presence of the disease under investigation Evidence-based Chiropractic

26. Case-control study design Evidence-based Chiropractic

27. Case-control studies are retrospective • Exposure levels are determined by looking back in time before the person became a case or a control subject • Because they are retrospective • Cannot determine the risk of developing a disease • But can estimate the odds of developing a disease given that a person was exposed to a risk factor; the odds ratio (OR) Evidence-based Chiropractic

28. Odds ratio (OR) • A ratio of the odds of exposure among cases divided by the odds of exposure among controls Evidence-based Chiropractic

29. 2X2 contingency table used to calculate OR May also be expressed ad/bc Evidence-based Chiropractic

30. Interpretation of ORs • OR = 1 • No association between an exposure and a disease • OR > 1 • A positive association between an exposure and a disease • The exposure is considered to be harmful • OR < 1 • A negative association between an exposure and a disease • The exposure is protective Evidence-based Chiropractic

31. Bias in case-control studies • This design is vulnerable to a number of biases because cases and controls are selected after the disease outcome and the exposure have occurred • Recall bias • Systematic differences between the way cases and controls recall past exposures Evidence-based Chiropractic

32. Bias in case-control studies (cont.) • Berkson’s bias (a.k.a, admission rate bias) • In a hospital setting, cases and controls are systematically different from each other • This is related to cases being more prone to hospital admission simply because they have a higher rate of exposure and incidence of the disease • It is actually a type of selection bias Evidence-based Chiropractic

33. Case-control studies and rare diseases • Case-control studies are usually the best way to study rare diseases • Definitive prospective studies typically require many subjects, which is not feasible if the disease is rare • It is much easier to select a group of subjects who already have a rare disease and then look back into their histories for clues Evidence-based Chiropractic

34. Advantages of case-control studies • Good for investigating rare diseases • Can be performed quickly and inexpensively • Useful for studying diseases with long latency period between exposure and manifestation • Facilitates the study of multiple potential causes at once • Existing records can often be used Evidence-based Chiropractic

35. Disadvantages of case-control studies • Typically rely on patients’ recall of past exposures • Does not permit calculation of true disease rates in the population • Difficult to validate information on exposure • Other variables that may be associated with the disease are not controlled Evidence-based Chiropractic

36. Confounding in case-control studies • Studies have shown that coffee drinking is positively associated with lung cancer • However, coffee drinkers are more likely to be smokers • When considered separately, there is no relationship between coffee drinking and lung cancer • Smoking is the culprit Evidence-based Chiropractic

37. Confounding in case-control studies (cont.) • Stratified analysis • The effect each of the independent variables on the outcome is analyzed separately • Commonly used to distinguish confounding variables from true exposure-disease associations • Data is divided into strata of homogenous subgroups to see if an association observed in the undivided group data remains Evidence-based Chiropractic

38. Categories of case-control studies • Prevalent case • All persons with the disease during the observation period are included in the study • For example, all low back pain patients at a chiropractic clinic between Jan. 1 and Dec. 31 • Cumulative incidence • Only new (incident) cases that occur during the period of observation are selected Evidence-based Chiropractic

39. Cohort studiesa.k.a., longitudinal studies • Studies that follow groups of subjects forward in time and compare their outcomes • One group is exposed to a known or suspected cause of disease while the other group is not exposed • Or one group was exposed to a risk factor in the past Evidence-based Chiropractic

40. Cohort studies (cont.) • A population is identified, from which two or more groups of subjects are selected • The groups are called cohorts • Outcomes are assessed on both groups prospectively over a defined time period • The objective is to determine if there are differences of disease occurrence between the groups Evidence-based Chiropractic

41. Cohort study design One cohort is exposed to a risk factor, while the other is not A population is identified Evidence-based Chiropractic

42. They have major advantage over case-control studies • Able to establish the temporal precedence of an exposure in relation to a health outcome • Thus, they satisfy one of the main prerequisites in determining causation • They are much less subject to bias because the exposure level is evaluated before disease develops Evidence-based Chiropractic

43. Advantages over case-control studies (cont.) • The best design to determine risk of a harmful substance • Still considered observational because random assignment to groups is not used • More useful investigating common diseases because of the complexity and expense of assembling cohorts that are large enough to investigate rare diseases Evidence-based Chiropractic

44. Advantages over RCTs • Cohort studies are easier to administer and less costly than RCTs • Usually more acceptable than RCTs from an ethical perspective • Potentially harmful treatments are not utilized • Treatment is not withheld from one of the groups Evidence-based Chiropractic

45. Advantages of cohort studies • Portrays the natural history of disease • Does not rely on patient recall • Better to establish a cause-effect relationship than case-control • Less vulnerability to bias or chance • Permits calculation of true disease rates in the population Evidence-based Chiropractic

46. Disadvantages of cohort studies • Typically very expensive • Many people must be followed to obtain enough with the disease • Very time consuming, given that one must wait for people to develop the disease • Subjects frequently drop out in time • Difficult to generate a control group to study very common conditions Evidence-based Chiropractic

47. Cohort studies are capable of determining risk • Relative risk (RR) is calculated in cohort studies • RR is the probability of disease in the exposed group, divided by the probability of disease in the unexposed group Evidence-based Chiropractic

48. RR is calculated using a 2X2 contingency table Evidence-based Chiropractic

49. Interpretation of relative risk • If RR is different from one, the risk factor is considered to be associated with the risk of disease • RR>1, the association is positive • RR<1, there is a negative or protective association • If RR=1, there is no association Evidence-based Chiropractic

50. Relative Risk example • Cohort study where 50 subjects exposed to dust (work in a coal mine) and 50 subjects work in offices – followed for 20 years • Exposed: 20/50 develop lung cancer • Not exposed: 5/50 develop the condition • RR=4 • Exposed were 4 times more likely to develop lung cancer Evidence-based Chiropractic