EFP 6 Flashcards
Define epidemiology
The study of the distribution and determinants of disease in human populations
Define Prevalence
A disease in a defined population at one point in time
(cross-sectional study)
- expressed as %
prevalence = All positive cases/ population
What is the point prevalence?
of cases on a specific point in time
What is period prevalence?
# of cases over a short period of time usually specifies a time frame (1988)
What is the incidence?
Number of new cases of a disease arising at a defined population over a period of time
- Expressed as a rate because it has a time frame and defined population
Incidence = new cases/ population at risk
What is the risk?
The probability of a disease occurring in a disease-free population during a specified time period
- expressed as period n/P (n=new case in x period, P=pop at risk)
assumes follow up in population don’t change with time
Risk= new cases in a period of time/ population at risk
What is the rate?
Probability of disease occurring in a disease-free population during the sum of individual time periods.
Specifies the time the population was at risk.
- Rate = n / total person-time of follow-up
What is the cumulative incidence?
measured by risk
population has been followed up by the total of the time specified
What is the incidence density?
measured by rate over the time people were followed up.
What is the rate/risk?
The numbers that apply to all the population
What is category-specific rate/risk?
Applies to specific population category (E.x. age-group)
Definition of ecological studies
Anecologicaloraggregatestudywhichfocuseson the comparison of groups, at a population-level rather than individuals
– Oftenroutinely-collect statistics
– e.g.infantmortalityrateofacountry
– Hypothesis-generating,not hypothesise-proving
Advantages and disadvantages of ecological studies
Advantages
– quick, inexpensive
Disadvantages
– inability to link exposure with a disease, inability to
control for confounding
– very poor evidence of cause and effect’
– subject to ecological fallacy-mistaken perception of
association between health statistics
Definition of cross-sectional studies
a sample of the population is selected and
information is obtained at one point in time
– questionnaires±examinations± investigations
– most common output: prevalence
Advantages and disadvantages of Cross-sectional studies
Advantages – measures prevalence (of exposure & disease ) in defined population – usuallycheap&inexpensive – good for exposures which never change
Disadvantages
– (greater)need for representative data, difficult to get
representative sample
– subject to bias
– very limited evidence of ‘cause and effect’
Absolute risk
Isolated measure of risk
**no association to an exposure
Ex. 5 people with HIV per 1000
Relative risk (RR) Risk ratio/ rate ratio
cause effect relationship between exposure and outcome
RR= Re/Ru
Interpretation: Compared to “unexposed”, “exposed” had RR (x amount) increase in risk of developing “Outcome”
Relative Exposure (RE)
The absolute risk amongst people exposed
This is a rate
Relative Unexposed (Ru)
Absolute risk amongst unexposed people
This is a rate
Hazard Ratio
estimate of the ratio of the hazard rate in the treated versus the control group.
Attributable risk (AR) risk difference/ rate difference
ndication of the absolute magnitude of change in risk of outcome, associated with exposure
AR= (Re-Ru)x100
Interpretation: “exposure” causes RA (xx) more cases of the “outcome” per 100 people per year
Attributable risk % (AR%)
AR%= ((Re-Ru)/Re)x100
interpretation: RA% of the risk of the “outcome” among sick people is due to the “exposure”
Population Attributable risk (PAR)
Indication of the additional or excess risk of the outcome in the population, due to the exposure
PAR = Rt - Ru
Interpretation: among the whole population, exposure causes PAR more cases of the outcome per 100 people per year”
Risk in the whole population (Rt)
Risk in the whole population both exposed and unexposed
Population Attributable risk % (PAR%)
Proportion of risk among whole population that is due to exposure
PAR%=[(Rt-Ru)/Rt] x100
Interpretation: PAR% of the risk of the outcome among the whole population is due to the exposure
