1. Epidemiology I

2. What common measures are used in the field of epidemiology?
What is epidemiology?
What common measures are used in the field of epidemiology?
What are the subject areas studied by epidemiologists?
How has epidemiology evolved over time? What is the current focus of epidemiology?
What are the health challenges of modern medicine (and focus of epidemiology)?

3. Definition of Epidemiology
Study of the distribution and determinants of diseases and injuries in human populations
Concerned with frequencies and types of injuries and illness in groups of people
Focus is not on the individual
Concerned with factors that influence the distribution of illness and injuries

4. Background Relatively new science – emerged in 19th century Today:
In strictest terms – study of epidemics
Today:
Concerned with epidemic disease and all other forms of illness and bodily injury
Cancer, heart disease
HIV/AIDS
Alcoholism, drug addiction
Suicide
Automobile accidents …

5. Relationship Between Clinical Medicine and Epidemiology
Focus in medicine is the individual patient
Community replaces the individual patient in epidemiology

6. Fundamental Assumptions in Epidemiology
Disease doesn’t occur at random
Disease has causal and preventive factors
Disease is not randomly distributed throughout a population
Epidemiology uses systematic approach to study the differences in disease distribution in subgroups
Allows for study of causal and preventive factors

7. Components of Epidemiology
Measure of disease frequency
Quantification of existence or occurrence of disease
Distribution of disease - three questions
Who is getting disease?
Where is disease occurring?
When is disease occurring?
Formulation of hypotheses concerning causal and preventive factors
Determinants of disease
Hypothesis are tested using epidemiologic studies

8. Progression of Epidemiologic Reasoning
1. Suspicion that a factor may influence occurrence of disease
Observations in clinical practice
Are HC providers seeing unexpected illness patterns in their patients?
Examination of disease patterns
Do subpopulations have higher or lower rates?
Are disease rates increased in the presence of certain factors?
Observations in laboratory research
Theoretical speculation
What theories can be generated from existing knowledge of disease prevention and causation models?

9. Progression of Epidemiologic Reasoning (Cont.)
2. Formulation of specific hypotheses
Based on suspicions concerning influence of a particular factor on disease occurrence
3. Conduct study
Hypotheses are tested to determine if statistical associations between factors and disease occurrence exist
Study population is assembled from individuals with disease or outcome of interest and an appropriate comparison group
Data is collected and analyzed

10. Progression of Epidemiologic Reasoning (Cont.)
4. Assess validity of association
Does the observed association really exist?
Is the association valid?
Are there alternative explanations for the association?
Chance
Bias
Confounding

11. Progression of Epidemiologic Reasoning (Cont.)
5. Make a judgement of whether a cause-effect relation between factor (exposure) exists
What is the magnitude of the association?
Are the findings consistent with previous studies (or conflicting)?
Are the findings biologically credible?
Can underlying biological mechanisms that support the association be identified?

12. Historical Perspective
Hippocrates - 5th century
Association between external environment and personal characteristics and health

13. “Whoever wishes to investigate medicine properly should proceed thus: in the first place consider the seasons of the year, and what effects each of them produces. Then the winds, the hot and the cold, especially such are as common to all countries, and then such as are peculiar to each locality. In the same manner, when one comes into a city to which he is a stranger, he should consider its situation, how it lies as to the winds and the rising of the sun; for it influence is not the same whether it lies to the north or the south, to the rising or to the setting sun. One should consider most attentively the waters which the inhabitants use, whether they be marshy and soft, or hard and running form elevated and rocky situations, and then if saltish and unfit for cooking; and the ground, whether it be naked and deficient in water, or wooded and well watered, and whether it lies in a hallow, confined situation, or is elevated and cold; and the mode in which the inhabitants live, and what are their pursuits, whether they are fond of drinking and eating to excess, and given to indolence, or are fond of exercise and labor.” (Hippocrates, “On airs, waters and places” Medical Classics 3:19, 1938).

14. Historical Perspective
John Graunt – 1662 (Hennekins and Buring 1987)
The Nature and Political Observations Made Upon the Bills of Mortality
Systematic statistical approach
Analyzed births and deaths in London
Excess of males born, higher mortality for males
Infant mortality is very high
Seasonal variation for mortality
Importance of routinely collected information for study of human illness
William Farr
Examined mortality and occupation and marital status
Identified important issues in epidemiological investigations
Use of comparison population, influence of multiple factors on disease

15. Historical Perspective
John Snow (1854) – Father of modern epidemiology
Established modern epidemiologic methods
Cholera epidemic in London
Plotted geographical location of all cases – deaths from cholera

16. From The Visual Display of Quantitative Data, Edward R. Tufte

17. John Snow (cont)
Went door to door, collecting information on daily habits
Suspected water supply as source of epidemic
Broad street pump closed, epidemic stopped
Mode of investigation – “shoe leather”
Practical application of epidemiology – use epidemiological investigation to impact a health problem

18. How the Epidemiologist Works
Studies origin and distribution of a health problem
Collection of data
Constructs a logical chain of inferences to explain the various factors in a society or segment of society that cause a health problem to exist
Likened to a detective investigating the scene of a crime looking for clues
Starts with examination of sick person(s)
Extends investigation to the setting where illness is occurring
Looks for common denominator that links all the affected so that the cause of the problem can be eliminated or controlled

19. Epidemiologic Analyses – Areas of Study
Causal agents related to disease:
Biological agents – bacteria, viruses, insects
Nutritional agents – diet (fats, carbohydrates, food nutrients)
Chemical agents – gases, toxic agents
Physical agents – climate, vegetation, chemical pollutants (air, water, food)
Social agents – occupation, stress, social class, lifestyle, location of residence

20. Epidemiologist studies:
Host characteristics:
Biological factors
Age, sex, degree of immunity, other physical attributes that promote resistance or susceptibility
Behavioral factors
Habits, culture, lifestyle
Social environment
Living conditions such as poverty, crowding
Norms, values and attitudes
Socially prescribed standards of living
Use of food and water, food handling practices
Household and personal hygiene

21. Eras of Epidemiology Sanitary era – early 19th century
Infectious disease era – between late 19th century and early 20th century
Chronic disease era – 2nd half of 20th century
Eco-epidemiology era – 21st century

22. Definitions

23. Case Episode of disorder, illness, or injury affecting an individual
Case of measles
Cancer case
TB case
Food poisoning event
Various sources provide case information
Interviews or surveys
Medical providers
Institutions or agencies

24. Incidence
Measure of new cases of disease (or other events of interest) that develop in a population during a specified period of time
E.g. Annual incidence, five-year incidence
Measure of the probability that unaffected persons will develop the disease
Used when examining an outbreak of a health problem

25. Prevalence Number of existing cases of disease or other condition
Proportion of individuals in a population with disease or condition at a specific point of time
Diabetes prevalence, smoking prevalence
Provides estimate of the probability or risk that one will be affected at a point in time
Provides an idea of how severe a problem may be – measures overall extent
Useful for planning health services (facilities, staff)

26. Epidemic, Endemic and Pandemic
Any significant increase in the number of persons affected by a disease
The first occurrence of a new disease
Endemic
A disease that is established within a population that remain at a fairly stable prevalence
Pandemic
Widespread, universal disease penetration over a wide geographic area

27. More Terms Morbidity – illnesses, symptoms, impairments
Mortality - deaths
Acute disease – diseases that strike and disappear quickly, within a month or so (chicken pox, colds)
Chronic disease – long term or lifelong diseases, incurable

28. More Terms Birth cohort Life expectancy (LE)
Persons born in a given year
Life expectancy (LE)
Average number of years of life remaining to a person at a particular age
Based on mortality rates and personal characteristics (e.g. gender, race)
Years of potential life lost (YPLL)
Measure of premature mortality
Death before age 75

29. Epidemiologic Measures

30. 1:1.1 ratio of female to male births
Used to compare two quantities
1:1.1 ratio of female to male births
Used to show quantity of disease in a population
cases
population

31. Proportion
A specific type of ratio in which the numerator is included in the denominator, usually presented as a percentage

32. Calculation of proportion:
Males undergoing bypass surgery at Hospital A
Total patients undergoing bypass surgery at Hospital A
352 males undergoing bypass surgery
539 total patients undergoing bypass surgery
65.3% =

33. Rate Special form of proportion that includes a specification of time
Most commonly used in epidemiology because it most clearly expresses probability or risk of disease or other events in a defined population over a specified period of time
3 major types
Crude rates
Specific rates (age-specific, infant mortality)
Adjusted rates

34. Crude rates Unadjusted, simple ratios Crude mortality rate:
cases in defined period of time
x K
population in defined period of time
(k denotes units 100’s, 1,000, etc.)
Crude mortality rate:
Total deaths in 2003
x 1,000 = U.S. death rate
Estimated U.S. pop in 2003

35. Calculation of rates: Number of events in a specified time period
Population at risk of these events in a specified time period
X k
k is used to denote the units of population such as per 1,000 or per 100,000
9,981 deaths in Detroit in 2000
951,270 total population in Detroit 2000
= per 1,000
1049 per 100,000

36. Detroit Population N=951,270 in 2000
9981 deaths
15,892 births
7,181 to single “named” parent
Is Detroit population declining, stable or increasing?

37. Specific Rates
Capture effects of specific variables or social characteristics
Age-specific, gender-specific, gender and race-specific
Example – infant mortality – deaths within the 1st year of life
Total # of deaths in 2003 among
persons age less than 1 year
x 1,000 = infant
Number of live births during mortality rate

39. Adjusted or Standardized Rates
Allow for comparison of populations with different characteristics
Statistically constructed summary rates allow for appropriate comparisons by taking into account differences in populations (age, gender, etc.)
Example of use: Population in Arizona is much older than population in Alaska, so it would be inappropriate to compare mortality rates. Standardization allows for meaningful comparisons.

40. Calculating prevalence:
Number of existing cases of disease
P = at a given point in time
Total population at risk
2176 DNW pts with asthma encounter
P = = .07
31005 DNW pts
= 7 asthmatics per 100 pts
= 7 %

41. Prevalence calculation exercise: Pediatric Asthma at DNW
Number of existing cases of disease
P = at a given point in time
Total population at risk
2159 DNW pts < 19 with asthma encounter
P = =
9173 DNW pts < 19 =

42. Types of Prevalence
Point prevalence: number of cases that exist at a given point in time
Lifetime prevalence: proportion of the population that has a history of a given disorder at some point in time
Period prevalence: number of cases that exist in a population during a specified period of time

43. Cumulative Incidence
The proportion of individuals who become diseased during a specified time period.
Time period can be a calendar year, 6 months, 3 years, 5 years, etc.

44. Formula for cumulative incidence:
Number of new cases of disease during a given time period
CI =
Total population at risk
70 new cases of breast cancer in a 5 year period
CI =
3,000 women at risk =
= 23 cases per 1,000 women during 5 years

45. Incidence Rate Also known as incidence density
Measure of incidence that is able to handle varying observation periods
Denominator is sum of person-time at risk

46. Formula for incidence rate or incidence density:
Number of new cases of disease during a given time period
ID =
Total person-time at risk
70 new cases of breast cancer
ID =
13,000 women-years of observation =
= 5.4 cases / 1,000 women years

47. Relationship Between Incidence and Prevalence
Prevalence varies directly with both incidence and duration.
If incidence is low, but duration is long (chronic), prevalence will be large in relation to incidence.
If prevalence is low because of short duration (due to recovery, migration or death), prevalence will be small in relation to incidence.

48. Special Types of Incidence Rates
Morbidity rate - number of nonfatal cases in the population at risk during a specified period of time
Mortality rate - number of deaths in a population at risk during a specified period of time
Cause-specific mortality - death from a specific cause
Case fatality rate - number of deaths from a disease divided by all case of the disease
Attack rate - cumulative incidence expressing the risk of disease among a population observed for a specified period of time

49. Special types of incidence and prevalence measures
Rate
Type
Numerator
Denominator
Morbidity rate
Incidence
New cases of nonfatal disease
Total population at risk
Mortality rate
Number of deaths from a disease or all causes
Total population
Case-fatality rate
Number of deaths from a disease
Number of cases of that disease
Attack rate
Number of cases of a disease
Total population at risk, for a limited period of observation
Disease rate at autopsy
Prevalence
Number of persons autopsied
Birth defect rate
Number of babies with a given abnormality
Number of live births

50. Measures of Association
Calculations used to measure disease frequency relative to other factors
Indications of how more or less likely one is to develop disease as compared to another

51. Two by Two Tables
Used to summarize frequencies of disease and exposure and used for calculation of association.
Disease
Yes No
Total a b
Yes
a + b
Exposure c d No
c + d
Total
a + c
b + d
a + b + c + d

52. Two by Two Tables: Contents of Cells
a = number of individuals who are exposed and have the disease
b = number who are exposed and do not have the disease
c = number who are not exposed and have the disease
d = number who are both non-exposed and non-diseased
***************************************************
a + b = the total number of individuals exposed
c + d = the total number of unexposed
a + c = the total number with the disease
b + d = the total number without the disease
a + b + c + d = sum of all four cells and the total sample size for the study

53. Relative Risk
Measure of association between incidence of disease and factor being investigated
Ratio of incidence rate for persons exposed to incidence rate for those not exposed
Incidence rate among exposed
RR =
Incidence rate among unexposed
Estimate of magnitude of association between exposure and disease

54. Formula for relative risk:
Incidence rate among exposed
RR =
Incidence rate among unexposed
a / (a + b)
RR =
c / (c+ d)
Risk ratio
If RR calculated from cumulative incidence
Rate ratio
If RR calculated from incidence rate (person units of time)

55. RISK RATIO: Example Breast No Breast Cancer Cancer Total
Alcohol , ,000
No alcohol , ,000
RR using Cumulative Incidence (CI):
a/(a + b) / 3,000
c/(c + d) / 3,000 =
= 1.4 =

56. Interpretation of Relative Risk
1 = No association between exposure and disease
Incidence rates are identical between groups
> 1 = Positive association
< 1 = Negative association or protective effect
Example: .5 = half as likely to experience disease

57. Odds Ratio Breast No Breast Cancer Cancer Alcohol 70 100
No alcohol
a x d (70) (140)
b x c (50) (100)
* Used for case control studies because persons are selected based on disease status so you can’t calculate risk of getting disease
OR = =
= 2.0

58. Difference Measures Attributable risk
# of cases among the exposed that could be eliminated if the exposure were removed
= Incidence in exposed - Incidence in unexposed
Population attributable risk percent
Proportion of disease in the study population that could be eliminated if exposure were removed
Incidence in total population - Incidence in unexposed
incidence in total population =

59. Impact of Modernization on Health
Infant mortality decreased
Life expectancy greatly increased during 20th century
Males
Increased from 48 to 74 years
Females
Increased from 51 to 79 years
Persons living longer with multiple illnesses
Chronic and degenerative diseases
Illness with social causes requiring social solutions

60. Changing Mortality Patterns
1900
Pneumonia/Influenza
Tuberculosis
Heart disease
Stroke
Diarrhea/enteritis
Nephritis
Cancer
Accidents
Diphtheria
1990s
Heart disease
Cancer
Stroke
Chronic lung disease
Unintentional injuries
Pneumonia/influenza
Diabetes mellitus
HIV/AIDS
Suicide

61. U. S. Life Expectancy Remaining life expectancy in years Males Females
1900 – at birth
46.3
48.7
1950 – at birth
65.6
71.1
1950 – at age 65
12.8
15.8
1999 – at birth
73.9
79.4
1999 – at age 65
16.1
19.1
1999 – at age 75
10.0
12.1
Health, United States, 2002

62. U.S. Infant Mortality Rate
Deaths per 1,000 infants
Significance:
Measure of society’s sanitary and medical standards
Health care
Diet
Living conditions

63. Factors Influencing Changing Pattern
Improvements due to industrialization
Nutrition
Environmental
Sanitation
Water supply
Housing
Medical advancements
Antibiotics
Immunization
Disease surveillance programs

64. Factors Influencing Changing Patterns
Problems associated with industrialization
Environmental pollutants
Increase in smoking
Excess consumption of calories and dietary fats
Lack of exercise, physical activity
Stress
Alcohol, drug use

65. Challenges of Modern Medicine
Behavioral aspects of health
Promotion of healthy lifestyles
Diet
Exercise
Tobacco, alcohol, drugs
Sexual behavior
Management of stress
New diseases – AIDS, SARS, West Nile Virus, bioterrorism