Lecture 3: Epidemiology: How do we measure disease? Flashcards
What is epidemiology?
Science of DISTRIBUTION and FREQUENCY of disease
What does Distribution of disease involve?
- Person
- Place
- Time
What does frequency of disease involve?
Incidence and Prevalence
What can epidemiology do
DEPC:
Describe, explain, predict, control
Describe in DEPC
Describe health of a population
Explain in DEPC
Explain etiology (cause) of disease
Predict in DEPC
Predict occurrence of disease
Control in DEPC
Control distribution of disease
What are the assumptions of epidemiology?
- Disease does not occur randomly
- Disease has causal and preventative factors that we can
identify through systematic investigation of different
people at different places or times
What are the 3 types of epidemiology?
- Descriptive
- Analytic
- Experimental
What is the goal of descriptive epidemiology?
To examine patterns of disease, health behaviours)
What is the goal of analytic epidemiology
To evaluate relationships between risk/protective factors and disease
What is the goal of experimental epidemiology?
To evaluate effect of treatment/intervention on disease
What is the foundation for all the types of epidemiology?
Foundation for all is being able to MEASURE PATTERNS (OCCURRENCE) OF DISEASE
What are the types of measures of disease occurrence?
- Rank
- Count(s)
- Ratio
- Proportion
- Rate
What is the key consideration for count, ratio, proportion, and rate?
Who, what is in numerator and/or denominator
What does rank measure?
Measures the order of disease
occurrence
ex. Arthritis and other rheumatic
conditions are a leading cause of
work disability among US adults
* Diabetes is the 7th leading cause of death in the US
What does count measure?
Measure(s) the number of
persons who have a given
disease (no denominator)
Ex. Number of female(s) with disease
X (red)
What is a ratio? What does it allow for?
The quotient of two numbers
- Numerator not necessarily included in denominator (no specific relationship between numerator and denominator)
- Allows comparison of quantities of different nature
- Ex. male to female ratio
Measures of association in epidemiologic students are?
Ratios
* Relative risk (RR) (or Rate ratio or Risk ratio)
* Odds ratio (OR)
What is a proportion?
- The quotient of 2 numbers
- Numerator is necessarily included in the denominator
- Expressed as a fraction, decimal, or percentages
What is rate?
- The quotient of two numbers
- Numerator
- Number of ‘events’ (new cases
of disease) observed for a given
time
- Number of ‘events’ (new cases
- Denominator
- Population in which events occur
- Includes time
- Ex. 1/10 = 0.1 per year
- 5-year cancer survival rate
- Ratio of people who are alive for 5
years after cancer diagnosis to
people in the general population
who are alive over the same 5 year
interval
What is prevalence?
- All individuals affected by disease at a particular time
- “Refer to individuals as “prevalent cases” or “active cases”
What is Incidence?
- New individuals with a disease during a particular period of time
- Refer to individuals as “incident cases” or “new cases”
Prevalence (a proportion) Definition
Proportion of a population affected by a disease at a specific time
Prevalence = Number of individuals with disease at specific time/Number of individuals in population
*Usually expressed as a percentage
What is point prevalence?
Proportion of a population affected by a disease at a point in time (e.g., prevalence of type 2 diabetes on December 31, 2020
What is period prevalence?
Proportion of a population affected by a disease during a certain period of time (e.g., prevalence of diabetes in 2020)
What is the definition of Incidence?
The number of new cases of a disease that develop over
time
What is Incidence proportion
- Cumulative incidence
Incidence proportion= Number of new cases of disease over period of time/Number of individuals at risk for disease at beginning of time period
- Ex. 4/10 = 0.4
40% (expressed as percentage) OR
= 4 cases per 10 population during 5 year period
= 0.8 cases per 10 population per year
= 8 cases per 100 per year
What is incidence rate?
- Incidence density
Incidence rate = Number of new cases of disease over period of time/𝚺 person-time
What does person-time from incidence rate mean?
Time each person was observed
Consider:
* 1 person at risk of a disease observed for 1 year = 1 person-year
* 1 person at risk of a disease observed for 1 month = 1 person-month
* Can be presented in many different ways:
* 10 cases per 1000 person-years
* 1 case per 100 person-years
* 0.1 cases per 10 person-years
* 0.01 cases per 1 person-year
Incidence proportion: Numerator
- New cases
Incidence proportion: Denominator:
Number of individuals at beginning of period
Incidence proportion: Time period
- Specified but not included in calculation
Incidence rate: Numerator
New cases
Incidence rate: Denominator
Person-time
Incidence rate time
Incorporates time directly into denominator
Prevalence “Disease Burden”
- Measure population disease status
- Help plan health care delivery
- Indicate groups who should be targeted
Incidence “Disease Risk”
- Assess frequency of disease onset
- Estimate risk of disease development
- Study risk factors for disease
- Evaluate preventive interventions
