Lecture 2 - Epidemiologic Indices

Made by Harley Chase and Kylee Wood

What do Epidemiologic Indices or Measures provide information regarding?

• The frequency of a disease or condition

• Associations b/w exposures and health outcomes

  • Strengths of those relationships

What is the Natural History of Disease synonymous with?

A disease’s ecology

What is the course of disease from its beginning to its final clinical endpoints?

Natural History of Disease

While complete understanding is not necessary for treatment and control of disease, what does it facilitate?

Better interventions

In regards to the Natural History of Disease - what are the phases?

• Exposure to pathogen

• Preclinical Phase → Biological onset of disease & symptoms appear

• Clinical Phase → Symptoms have appeared, Diagnosis, Therapy has begun

• ‘Outcomes’ → cured, living w/ disease, deteriorated, died

  • Possible relapse & change in therapy

What are those members (animal or human) of the overall population who are capable of developing the disease or condition being studied?

Population at Risk (PAR)

T or F: Population at Risk is an EXTREMELY important concept in epidemiology.

True

How is the Population at Risk usually identified in rate calculations?

As the ‘denominator’

What are 3 common epidemiologic measures that are all ratios?

• Proportion

• Percentage

• Rate

What is defined as “The value obtained by dividing one quantity by another“?

Ratio

What are examples of a Ratio?

• Rates

• Proportions

• Percentages

What does a ratio in epidemiology usually imply?

That the numerator is not a subset of the denominator (but there are exceptions)

What is a type of ratio in which the numerator is part of the denominator?

Proportion

How is a proportion commonly expressed?

As a percentage

What are examples of a Proportion?

• Incidence (# of new cases)

• Prevalence (# cases)

• Case Fatality (# deaths)

What is a proportion that has been multiplied by 100?

Percentage

What is the Percentage formula?

(A / A+B) x 100

What is also a type of ratio, but differs from a proportion because the denominator involves a measure of time?

Rate

What are some common rates in epidemiology?

• Incidence Rate (# new cases / PAR)

• Prevalence Rate (# cases / PAR)

• Case Fatality Rate (# death / PAR)

What refers to the occurrence of new disease or mortality within a defined period of observation (e.g., a week, month, year, or other time period) in a specified population?

Incidence

In regards to Incidence - are cases that exist at the beginning of the period counted?

No

What is a rate formed by dividing the number of new cases that occur during a time period by the number of individuals in the population at risk?

Incidence Rate

What is probably the most common frequency or risk measure?

Incidence Rate

What is Incidence Rate used to roughly compare?

Risk between similar populations

What is the Incidence Rate Formula?

Incidence Rate = (# of new cases in a time period / total pop. at risk) x 10^n

• ^n can be any multiplier necessary to make the # manageable

What is the number of existing cases of a disease or health condition, or deaths in a population at some designated time (regardless of when they started)?

Prevalence

What are the variations of Prevalence?

• Point Prevalence

• Period Prevalence

• Lifetime Prevalence

What is the difference in Prevalence vs. Incidence?

• Prevalence → the # of existing cases of disease in the pop. during a defined period

• Incidence → the # of new cases of disease that developed in the pop. during a defined period

T or F: Incidence and Prevalence are different…but still related.

True

What are factors that cause prevalence to increase?

• Increase in incidence

• Longer duration of the case

• In-migration of cases

• Prolongation of life of patients without a cure

What are factors that cause prevalence to decrease?

• Decrease in incidence

• Shorter duration of disease

• In-migration of healthy people

• Improved cure rate of disease

What are the following factors responsible for?

• Increase in incidence

• Longer duration of the case

• In-migration of cases

• Prolongation of life of patients without a cure

Increase in Prevalence

What are the following factors responsible for?

• Decrease in incidence

• Shorter duration of disease

• In-migration of healthy people

• Improved cure rate of disease

Decrease in Prevalence

What is basically an incidence rate that has been applied to a narrow population over a narrow time period?

Attack Rate

What is the Attack Rate formula?

AR = (# new cases in the time period / Pop. at risk at beginning ) x 10^n

How is the Attack Rate usually expressed?

As a percent (n=2)

Where is Attack Rate commonly used? Especially when dealing with what?

In epidemics - especially foodborne illnesses

What is the numerator for Morbidity Rate?

The total # of illnesses in a population over a specified period of time

What is the denominator for Morbidity Rate?

The average population at risk over that same time period specified in the numerator

What is the Morbidity Rate formula?

Morbidity Rate = (# of illnesses due to the disease in the time period / avg. # in pop. during the time period) x 10^n

What is the numerator for Mortality Rate?

The total number of deaths in a population over a specified period of time

What is the denominator for Mortality Rate?

The average population at risk over the same time period specified in the numerator

What is the Mortality Rate formula?

Mortality Rate = (# of deaths due to the disease in the time period / avg. # in pop. during the time period) x 10^n

What is the number of deaths due to a disease that occur among persons who are afflicted with that disease?

Case Fatality Rate (CFR)

What is the Case Fatality Rate (CFR) formula?

CFR(%) = (# of deaths due to disease “X” / # of cases of disease “X”) x 100 during a time period

In the US, health officials get very concerned about diseases with a Case Fatality Rate (CFR) of what?

About 2% or more

What is a characteristic of value in predicting risk?

Risk Factor

What breeds have typical risk factors for degenerative joint disease associated with hip dysplasia?

• German Shepherd Dogs

• Golden Retrievers

• Labrador Retrievers

• Rottweilers

What are some different Risk Factors?

• Breed

• Sex

• Age

• Weight

• Activity

• Nutrition

• Genetics

What are the 3 types of Risk?

• Absolute Risk

• Relative Risk

• Attributable Risk

What means the rate of occurrence and is the same as Incidence?

Absolute Risk

What is (Incidence Rate Among Exposed / Incidence Rate Among Not Exposed)?

Relative Risk

What is Incidence Rate of Exposed minus Incidence Rate of Unexposed?

Attributable Risk