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HSS4303B – Introduction to Epidemiology

Epidemiologists: “Are they smart and good looking, or good looking and smart?" Discuss. HSS4303B – Introduction to Epidemiology. Jan 14, 2010 – Measures of Morbidity & Mortality.

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HSS4303B – Introduction to Epidemiology

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  1. Epidemiologists: “Are they smart and good looking, or good looking and smart?" Discuss. HSS4303B – Introduction to Epidemiology Jan 14, 2010 – Measures of Morbidity & Mortality

  2. Smoking more than one pack of cigarettes a day increases the risk of cardiovascular disease by 70%-80%, and smoking more than two packs per day increases the risk by 200%. Passive smoke increases the risk by 30%. -Basic Res Cardiol 2000 (Suppl I); 95:152-158 normal rate of miscarriage of pregnancy is 2 to 3%, and amniocentesis increases that risk by an additional 1/2 to 1%. -ds-health.com A 1-month delay in treatment for an early-stage primary breast cancer with a 130-day doubling time increases the risk of axillary lymph node involvement by 0.9% -Obstet Gynecol. 1996 Mar;87(3):414-8.

  3. A measure of the occurrence of new cases of the disease of interest in a population ? The probability of obtaining an outcome, given the presence or change in status of an exposure

  4. Risk is a measure of the occurrence of new cases of the disease of interest in a population • new cases A • Risk (R) = ----------------- = ---- • persons at risk N • Risk has no units and lies between 0 (no new cases) and 1 (the entire population has the disease of interest) • Risk can be expressed as a percentage also • If one person in a group of six gets flu the risk of getting flu in that group is • R= 1/6 =0.17 =17%

  5. In 1022 cancer patients with fever and granulocytopenia, 530 patients developed clinically or microbiologically documented bacterial infections What is the risk of infection in granulocytopenic febrile cancer patients? 530/1022 = 0.518 = 51.8%

  6. A recent study found that 30% of women who date online have had sex on the first date with gentlemen they've met online. Moreover, the study found that 77% of these women had had unprotected sex in those encounters. -www.onlinedatingmagazine.com/news2007/womenonlinedatersrisky.html "When you have unprotected sex with people you are meeting online, you are playing russian roullette [sic] with your health. It's not a matter of 'if' you'll get a sexually transmitted disease, but rather 'when' and 'how many'.” -’STI expert’ responding to the results of the study

  7. How would you go about calculating the actual risk of a heterosexual Canadian woman contracting an STI fro unprotected sex with a random Canadian man? What would you need to know? • Prevalence of STIs in Canada (among whom? Which diseases?) • What about transmission rate? (Above only computes risk of exposure)

  8. Canada’s STI Surveillance Report: www.phac-aspc.gc.ca The current prevalence is164 cases per 100,000 population, or about 0.16% of the total population, assuming a conservatively estimated base population of 35 million But is the right prevalence statistic to use?

  9. The age-specific chlamydia burden among Canadian men aged 25+ is 9374 cases. Canadian adult male population of about 20 million Risk of Canadian male having Chlamydia = 9374/20000000 = 0.00047 = 0.05% Is that the end of the story?

  10. Transmission So far we've been talking about the chances of being exposed to an STI. What about actually contracting one? The transmission rate of Chlamydia is between 30% and 40%. In other words, only 30-40% of sexual encounters with an infected person will result in the disease being transmitted. 30% of 0.05% = 0.015% 40% of 0.05% = 0.020%

  11. So, assuming the more conservative estimate (0.02% chance of both exposure and transmission), a Canadian woman would have to sleep with 5000 men to get anything resembling the “guarantee” of an STI that the “expert” suggested. What is wrong with this analysis?

  12. Two Terms: • Mortality • Morbidity

  13. Two Terms: • Mortality • Rate of death (due to a disease) • Morbidity • Rate of presence of a disease in a population • A diseases state

  14. Measurements of Morbidity • Incidence • And “cumulative” incidence • Prevalence • And “point” prevalence • Rate of new cases • Rate of all cases

  15. Incidence • Is the number of new cases of a disease that occur during a specified period of time in a population at risk of developing the disease • Incidence measures risk in specific groups of people (sex, age, occupation, ethnic background) • The denominator includes population at risk • Time must be specified and all individuals in the denominator must be at risk during that period of time • Time is arbitrary and depends on disease (week, month, year or a decade) • Can be expressed as a % or as a rate per 1000 people or in person-time (e.g., person-years)

  16. Incidence • In general, incidence is: A = number of new cases ------- P =population at risk Give over a specific time period

  17. Cumulative incidence vs Incidence Density • Cumulative incidence (also called incidence proportion) is incidence calculated using a period of time during which all of the individuals in the population are considered to be at risk for the disease • Incidence density (also called incidence rate) is calculated by including in the denominator the sum of time during which the individual was at risk (person years)

  18. Incidence Density A = number of new cases ____________ P T = population at risk (P) duration of risk (T) Given as a rate per population-time E.g. cases per 1000 person-years

  19. Incidence • Raywatville has 1000 people. Over a two year period, 28 people develop dumb-ass disease. What is the incidence of dumb-ass disease in Raywatville over this period? IR = 28/1000 = 28 cases per 1000 population or = 2.8% Is this cumulative incidence or incidence density?

  20. Incidence • Raywatville has 1000 people. Over a two year period, 28 people develop dumb-ass disease. What is the incidence of dumb-ass disease in Raywatville over this period? IR = A/PT = 28/(1000x2) = 14 per 1000 person-years Is this cumulative incidence or incidence density?

  21. Incidence density = A _______ P x T = A cases ______________ P persons x T time = 28 cases ---------------------- 1000 persons x 2 years = 14 cases ---------- person-years

  22. Example • A total of 5031 patients were observed for a total period of 127,859 patient-days • 596 patients developed a nosocomial infection • What is the incidence rate of nosocomial infection in the hospital? (Give it as number of cases per 1000 patient-days) 4.7 cases / 1000 patient-days Is this cumulative incidence or incidence density?

  23. Another Example • In the United States, the National Cancer Institute maintains a network of registries that collect information on all new occurrences of cancer within populations residing in specific geographic areas. • Collectively, these registries cover about 14% of the population of the United States, and between 1996 and 2000, 2957 females were newly diagnosed with acute myelocytic leukemia in these areas. • An estimated 19,185,836 females lived in these combined areas on average during this 5-year period. • First, what is the total number of woman-years for this period? 19,185,836 women x 5 years = 95,929,180 woman-years. Second, what is the incidence rate for leukemia for this period for this population? IR=A/PT = 2957/95929180 = 0.03 cases per 1000 woman-years

  24. Incidence rates Six patients were observed for 8 years. During that time, 2 were diagnosed with dumbass disease How would you go about computing the incidence rate of dumbass disease over this 8 year period?

  25. Incidence rates

  26. Incidence rates

  27. Incidence rates Incidence rate = A / PT A = 2 PT = 2+2+3+7+2+6 = 22 2 / 22 = 0.09 cases / person-year 9 cases / 100 person-years

  28. Acute hepatitis incidence in Canada

  29. Incidence rate calculation for a big population • Calculation of incidence rates for a large population, such as that in a city, by separately enumerating the person-years at risk for each individual, would require a tremendous amount of work. Fortunately, person-time for a large population can often be calculated by multiplying the average size of the population at risk by the length of time the population is observed: • PT = (Average size of population at risk) x (Length of observation period) • In many instances, relatively few people in the population develop the disease, and the population undergoes no major demographic shifts during the time period of observation. In such situations, the average size of the population at risk can be estimated by the size of the entire population, using census or other data. The person-time of a large, stable population can often be estimated by • PT = (Size of entire population) x (Length of observation period)

  30. Identifying new cases: step 1

  31. Identifying new cases: step 2

  32. Why is incidence important?

  33. Expected and observed new cases

  34. Prevalence

  35. Prevalence Rate • The number of affected persons present in the population at a specific time divided by the number of persons in the population at that time • Measures disease burden (diseases with long morbidity have higher prevalence) • Point prevalence measures prevalence at a certain point • Period prevalence measures prevalence during a certain period in time

  36. But you can also present it as a percent

  37. Or… PR = A ----------------- A + B A = number of cases of the disease B = number of people in the population who do not have the disease, but who are at risk for getting it. Therefore A+B = total number of people in the population

  38. Example: Obesity in the USA (from CDC.gov)

  39. “Point” Prevalence • the proportion of people in a population who have a disease or condition at a particular time, such as a particular date • A “snap shot” • PR = number of cases on a specific date ---------------------------------------------- number of popn at risk on that date

  40. “Period” Prevalence • the proportion of people in a population who have a disease or condition over a specific period of time, say a season, or a year. • PR = number of cases in that period ------------------------------------------------------------------ number of popn during that period

  41. Point prevalence Period prevalence 23% of this class is left handed In 2008, 28% of Americans were clinically obese

  42. Consider an old film camera… A single click of the camera produces an image. The total photons on the film, or dots on the image, constitute the point prevalence. If you leave the shutter open for a few seconds, and point the camera to an unmoving object, then many photons accumulate on the film. This is period prevalence. If you use a movie camera and analyze individual frames, then the number of new events in each frame constitute the incidence rate for that frame.

  43. Incidence and prevalence

  44. Relationship between incidence and prevalence • Prevalence = incidence x duration of disease • When rates are stable • When in-migration = out-migration • Rates and proportions • Spatial distribution

  45. Point prevalence for positive x-rays

  46. Breast cancer incidence

  47. What are the pitfalls of finding Inc/Prev Data? • Biases?

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