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Descriptive Epidemiology

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  1. Descriptive Epidemiology D. Morse

  2. Descriptive Epidemiology Describes the distribution of disease in human populations

  3. Uses of Descriptive Epidemiology For epidemiologists • provides clues about possible disease determinants (aka, risk factors)  hypothesis generation For public health administrators • identification of populations with high/low levels of disease or disease risk • allows for efficient allocation of resources • e.g., target subpopulations for prevention programs

  4. The five leading causes of death, United States, Males & Females, All ages, 1999 Death rate per 100,000 population Source: CA Cancer J Clin 52, 2002

  5. Leading site-specific causes of cancer death, United States, Males, 1999 Number of Deaths (x1000) Source: Ca Cancer J Clin, 2002

  6. Leading site-specific causes of cancer death, United States, Females, 1999 Number of Deaths (x 1000) Source: Ca Cancer J Clin, 2002

  7. Descriptive epidemiology describes the distribution of disease in terms of: • Person • Place • Time

  8. Person • Addresses “Who is getting the disease?” • Considers • demographic factors • socioeconomic factors • lifestyle factors

  9. Invasive Lung & Bronchus CancerAge-specific Incidence Rates by Gender, SEER Sites, USA, 1994-98

  10. Coronary Heart DiseaseAge-Specific Mortality Rates by Cigarette Smoking Status, Cancer Prevention Study II, 1982-88, Men

  11. Percentage of population reporting 1+ recurrent aphthous ulcers in past 12 months, United States Source: NCHS, 1996 Age

  12. Place • addresses “Where are disease rates highest/lowest?” • geographic location • country • region • urban/rural • includes migration studies

  13. Age-standardized incidence rates per 100,000 for stomach cancer, selected geographic areas, 1988-92 10 5 5 10 Source: Parkin et al., 1997

  14. Time • Addresses: • “When does the disease occur?” • “Is the disease frequency different from times past?”

  15. Lung & Bronchus Cancer (Invasive)Age-Adjusted Incidence Rates by Race, Gender SEER Sites, All Ages, 1973-98

  16. Incidence Rates for Tuberculosis, United States, 1992-99 Source: MMWR 53:83, 2001

  17. Sources of Data • Wide-ranging, including: • Census data • Vital statistics records • Clinical records from hospitals • National figures on • use of various products • apparent food consumption • Health surveys • Range: national, regional, communities, subpopulations

  18. Descriptive Studies, Major types: • Correlational studies • Case reports / Case series • Cross-sectional Surveys

  19. Correlational studies • use group or populationdata • correlatedisease frequency in • different groups at the same time • the same populationover time with some other factor of interest such as • consumption of a food, medication

  20. Correlational studies,Strengths • data often readily available • can be done relatively • quickly • inexpensively

  21. Correlational studies,Limitations inability to • link exposure with disease in particularindividuals • control for confounding • disentangle complicated relationships N.B., The presence of a correlation does not imply a valid statistical association The lack of a correlation does not imply the lack of a valid statistical association

  22. Case Reports / Case Series • Use data from • 1 person (case report) • 2+ persons (case series) • detailed reports of unusual medical/dental occurrences

  23. Case Reports/Case Series,Strengths Recognition of • new diseases / epidemics • new therapies • adverse exposures

  24. Case Reports/Case Series, Limitations Findings may be coincidental • CR:only one case • CS:no comparison group

  25. Weaver et al., Mouthwash and oral cancer: carcinogen or coincidence? J Oral Surg 37: 250, 1979. • Identify 11 nonsmoking, nondrinking patients from among 200 head & neck cancer cases • 10/11 used mouthwash  2x/day for 20+ years • most used a brand containing 25% alcohol, most used undiluted mouthwash • another 6 patients used alcohol and/or tobacco “only occasionally”… & “all used mouthwash excessively”

  26. Billings et al., Contemporary treatment strategies for root surface dental caries. Gerodontics 1: 20-7, 1985. Subjects • 6 patients (UTDB) • Age range: 31-71 years • total of 20 incipient root caries lesions Methods • 1% NaF gel via trays (5 min) • examined at 6, 12, 24 months Results (incipient root caries findings) Exam Period Active Arrested Baseline 20 0 6 months 15* 2 12 months 12 5 24 months 3 14 * 3 progressed and were restored

  27. Billings et al., Contemporary treatment strategies for root surface dental caries. Gerodontics 1: 20-7, 1985. Authors’ conclusion: “Within the limitations of this project, it can be concluded that incipient root lesions require no invasive or restorative intervention and will arrest in time when treated with topically applied NaF on a daily basis.” N.B., • 3/20 lesions progressed • lack of control series limits study’s interpretability / utility

  28. Cross-Sectional Surveys • use data from individuals • diseaseand exposure status are assessed simultaneously (snapshot in time) • data sources range from • national probability surveys to • small surveys in small sub-populations (e.g., nursing home) • time window • period of time (e.g., calendar year) • point in time

  29. Cross-Sectional Surveys,Strengths • furnish a “snapshot” of a population’s health experience at a specified time • provide information on • the prevalence of health outcomes in certain groups (e.g., occupational groups) • exposure patterns in certain groups

  30. Cross-Sectional Surveys, Limitation Generally, not possible to determine whether exposure preceded or resulted from disease Exception: when current value of exposure does not change over time (e.g, eye color)

  31. KEOHS Project, Kungholmen, Sweden, - Morse, Comm Dent Health, in Press Design: • Cross-sectional study • 129 dentate, community-dwelling residents of Kungsholmen • Age: 81-95 years • Caries examination (coronal & root) Findings (one of many): Relationship between active coronal and root decay % persons with coronal caries: - among persons with root decay 62% - among persons without root decay 22% (p<0.01) - similar findings after controlling for: age, gender, # teeth, # crowns

  32. KEOHS, Kungholmen, Sweden Interpretation: • Having active root caries is related to having active coronal decay in this population of older adults • Finding may or may not be generalizable to other populations of older adults • We cannot say • whether the coronal or root caries came first • that coronal caries caused the root caries (or vice versa)

  33. Prevalence versus Incidence

  34. Prevalence • also termed prevalence rate • relates to existingdisease • quantifies the proportion of persons in a population having the disease at a specific period or point in time • provides estimate of the probability (risk) that a person will be ill at that period or point in time

  35. Prevalence Number of existing cases of disease P = at a given point1/period2 in time Total population 1 Point prevalence 2 Period prevalence i.e., prevalence quantifies the proportion of persons in a population having the disease at a specific period or point in time

  36. Bowirrat A et al. Prevalence of Alzheimer’s type dementia in an elderly Arab population. Eur J Neurol 8:119, 2001. Methods: • Obtain a list of all persons aged 60+ (on 10/5/95) who lived in one of 3 Arab Israeli Villages • Door-to-door survey • Participants interviewed, examined • Dementia if DSM-IV criteria fulfilled • Exclude dementias likely due to other causes (e.g., vascular dementia, Parkinson’s Disease) Results: No. diagnosed with DAT: 168 No. persons examined: 821 (of 853 eligible identified on list) Crude prevalence rate: existing cases / total population 168 / 821 = 20.5%

  37. Bowirrat A et al. Prevalence of Alzheimer’s type dementia in an elderly Arab population. Eur J Neurol 8:119, 2001. Results (cont’d): Age-specific prevalence: Age DAT Examined Prevalence (%) 60-64 10 186 5.4 65-69 14 189 7.4 70-74 35 174 20.1 75-79 37 122 30.3 80-84 26 72 36.1 85+ 46 76 60.5

  38. Incidence • relates to new disease • quantifies the number of new cases of disease that develop in an at-risk population during a specified period of time or per unit of person-time • two types of incidence measures • Cumulative incidence (CI) • Incidence density (ID) [not covered in course]

  39. Incidence (Cumulative Incidence) Number of new cases of diseaseduring a given period of time CI = Total population at risk Incidence quantifies the number of new cases of disease that develop in an at-risk population during a specified period of time or per unit of person-time N.B., CI provides an estimate of the probability, or risk, that a person will develop a disease during a specified period of time.

  40. Cumulative Incidence - Example Invasive cancer of the bronchus, lung (ICD-9 162) Louisiana, New Orleans: Black (Males), All Ages, 1988-92* No. new cases (in 1990): 168 cases Population (1990 US Census) 179,523 males Crude CI = 168 cases in 1990/ 179,523 persons (males) at risk = 93.6 cases per 100,000 persons at risk in 1990 *N.B., Among the highest rates of bronchus/lung ca in World Source: Cancer Incidence in Five Continents Vol VII, 1997

  41. Relationship between Prevalence and Incidence Prevalence depends on • incidence rate • duration of disease P = I * D where I: incidence rate, D: mean duration N.B., in prevalence could result from:  incidence  disease duration Both

  42. Different ways to present rates • Category specific-rates • Crude rates • Adjusted (summary) rates

  43. Category-specific rates Rates presented for specific categories e.g., • age-specific rates • gender-specific rates Number of new cases of disease in a specific category+ CI = At-risk population in that specific category + during a specified period of time

  44. Bowirrat A et al. Prevalence of Alzheimer’s type dementia in an elderly Arab population. Eur J Neurol 8:119, 2001. Results: Age-specific rates Age DAT Examined Prevalence (%) 60-64 10 186 5.4 65-69 14 189 7.4 70-74 35 174 20.1 75-79 37 122 30.3 80-84 26 72 36.1 85+ 46 76 60.5