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. Dr. John Snow the father of epidemiology • Dr. Snow lived in London during a severe cholera epidemic • The medical establishment thought cholera was spread by miasmas (bad air from decayed organic matter) • But Dr. Snow theorized that it was spread by contaminated water Evidence-based Chiropractic

6. The father of epidemiology (cont.) • He surveyed the houses near a certain public water pump and discovered that nearly all persons who died had consumed water from that pump • He presented his data to the local authorities who immediately had the pump handle removed • The outbreak quickly subsided as a result Evidence-based Chiropractic

7. The father of epidemiology (cont.) • His theory continued to be rejected by his peers until Vibrio cholerae was eventually identified as the cause of the disease • However, the statistical and mapping methods used during the investigation formed the foundation of epidemiology Evidence-based Chiropractic

8. Dr. Snow’s map Evidence-based Chiropractic

9. Close-up of Dr. Snow’s map Broad street pump Deaths Evidence-based Chiropractic

10. Measurement of disease frequency and occurrence • Incidence • The probability of a person being diagnosed with a disease during a specific period of time (typically one year) • The number of newly diagnosed cases of a disease during a specified time period Evidence-based Chiropractic

11. Incidence (cont.) • For example, a study of 1,000 bus drivers found 50 new cases of low back pain in one year • The incidence is 50/1,000 = 0.05 or 5,000 per 100,000 • Some workers may have had more than 1 episode of low back pain during the year, but only the first occurrence is usually counted Evidence-based Chiropractic

12. Risk • An estimate of the proportion of unaffected persons in a population who will develop the disease of interest over a specified period of time • Estimated by observing a population that is at risk over a defined period of time to determine the number of new cases as compared with the total number of persons Evidence-based Chiropractic

13. Prevalence • The proportion of persons in a given population that have a disease at a certain point in time • The total number of cases of the disease in that population • It does not matter when they were diagnosed with the condition Evidence-based Chiropractic

14. Prevalence (cont.) • For example, a study of community with a population of 30,000 which found 1,000 persons with migraine headaches • The prevalence is 1,000/30,000 = 0.33 or 3,333 per 100,000 persons • It doesn’t matter that 200 of the cases were newly diagnosed and 800 already had migraine headaches when the study began Evidence-based Chiropractic

15. Point prevalence • The proportion of a population with a disease at a given point in time • May underestimate the frequency of certain conditions Evidence-based Chiropractic

16. Period prevalence • The proportion of a population that has a disease within a defined period of time • Involves repeated monitoring of a population • Thus is a better depiction of the overall frequency of a disease Evidence-based Chiropractic

17. Incidence and prevalence • Sometimes reported as a percentage • For example • Manchikanti et al studied 500 consecutive chronic spine pain patients and reported the prevalence of facet joint pain as 55% in the cervical, 42% in the thoracic, and 31% in the lumbar spine Evidence-based Chiropractic

18. Incidence and prevalence (cont.) • Prevalence is usually much lower than incidence in short duration diseases (e.g., the common cold) • Many people contract colds each year, but relatively few people have a cold at one time because the duration is so short Evidence-based Chiropractic

19. Incidence and prevalence (cont.) • Prevalence in chronic conditions is higher than its incidence (e.g., diabetes) • There are 1.2 million new cases of diabetes each year, but once diagnosed, the condition remains and each year’s incidence is added to the overall prevalence, minus those who die having the condition = 18.2 million Evidence-based Chiropractic

20. Case begins Case ends Evidence-based Chiropractic

21. Causation in epidemiology • In order to determine whether exposure to a specific risk factor actually caused a particular disease within a population, three key criteria should be met • Temporality • Consistency • Dose-response Evidence-based Chiropractic

22. Temporalitya.k.a., temporal precedence • The exposure must occur prior to the onset of a disease • However, just because a given exposure precedes a given disease, does not necessarily mean there is a cause-and-effect relationship • Alternate explanations are possible Evidence-based Chiropractic

23. Temporality (cont.) • Four possible interpretations of temporal relationships • Event A caused event B • Event B caused event A • Both events A and B were caused by a third related event • Neither A or B are related to each other or a third event, but the temporal relationship was merely by chance Evidence-based Chiropractic

24. Consistency • Studies on the relationship carried out by other researchers using different populations get similar results • A large number of good quality observational studies reporting consistent results are required in order to conclude that A caused B Evidence-based Chiropractic

25. Dose-response • When greater exposure to a risk factor results in a greater effect on health • For example, the more cigarettes a person smokes per day (the dose), the more likely they are to develop lung cancer (the response) Evidence-based Chiropractic

26. Bradford Hill’s criteria for causation • Six additional criteria used to identify cause-and-effect relationships • Keep in mind, none of these criteria can bring indisputable evidence for or against a cause and effect relationship Evidence-based Chiropractic

27. Bradford Hill’s Criteria of Causation • Strength of association – The stronger the relation between the risk and the outcome is, the less likely it was caused by other factors • Consistency – The replication of study results by different researchers in a different setting • Specificity in the cause – The exposure should be associated with a single specific disease • Temporality – The exposure must precede the disease • Dose response relationship – Increased exposures should correspond to increased risk of disease • Plausibility – There should be a rational scientific basis for the association risk of disease • Coherence – The association must be consistent with other knowledge on the topic • Experimental evidence – Research that is based on experiments reinforces a causal inference • Analogy – The association is analogous to a known causal relationship Evidence-based Chiropractic

28. 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

29. 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

30. 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

31. 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

32. 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

33. 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

34. 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

35. 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

36. 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

37. 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

38. 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

39. 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

40. 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

41. 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

42. 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

43. 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

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

45. 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

46. 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

47. Case-control study design Evidence-based Chiropractic

48. 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

49. Odds ratio (OR) • The ratio of the odds of developing the disease in the exposed group divided by the odds of developing the disease in the unexposed group Evidence-based Chiropractic

50. 2X2 contingency table used to calculate OR Evidence-based Chiropractic