Epidemiology Flashcards
What is the definition of epidemiology?
The study of the frequency, distribution and determinants of diseases and health-related states in populations in order toe prevent and control disease
What is the incidence?
Number of new cases; the denominator is the number of disease free people at the start of the study; it is over time
What is the prevalence?
Number of existing cases; it is at a point in time
What is person-time?
Person-time is a measure of time at risk. i.e. time from entry to a study to (i) disease onset, (ii) loss to follow-up. (iii) end of study
What is person-time used for?
It is used to calculate incidence rate which uses person-time as the denominator. (cumulative incidence uses the number of disease free people at the beginning of the study)
What is incidence rate used for?
It is useful when study participants are followed up for varying lengths of time
What is the incidence rate?
Number of persons who have become cases in a given time period / Total person-time risk during that period
What are the usual headings when describing the epidemiology of a disease?
Time; Place; Person (age, gender, class, ethnicity)
What is absolute risk?
It has actual numbers and units, so per number of people
What is relative risk?
Risk in one category relative to another
What is the attributable risk?
The rate of disease in the exposed that is due to the exposure
What does the relative risk tell us about?
About the strength of association between a risk factor and a disease
What does the attributable risk tell us about?
About the size of effect in absolute terms
What is bias?
A systematic deviation from the true estimation of the association between exposure and outcome
What are the two main types of bias?
Selection bias and information bias
What is a systematic bias?
An error in the selection of study participants or the allocation of participants into different study groups
What is an information bias?
A systematic error in the measurement or classification of exposure or outcome.
What are the sources of information bias?
Observer (observer bias); participant (recall bias); instrument (wrongly calibrated instrument)
What is confounding?
The situation where a factor is associated with the exposure of interest and independently influences the outcome (but does not lie on the causal pathway)
What should you consider when looking at association and causation?
Bias, chance, con-founders, criteria
What are the factors when assessing causality?
Strength of association; Dose-response; Consistency; Temporality (does exposure come before outcome); Reversibility (if you take away the exposure the outcome goes away); Biological plausability
How can you classify prevention?
Primary, secondary and tertiary
What is primary, secondary ad tertiary prevention?
Primary - no disease
Secondary - pre-clinical disease
Tertiary - clinical disease
What is the population approach to disease prevention?
A preventative measure delivered on a population wide basis and seeks to shift the risk factor distribution curve
What is the high risk approach to disease prevention?
It seeks to identify individuals above a chosen cut-off and treat them
What is the prevention paradox?
A preventive measure which brings much benefit to the population often offers little to each participating individual.
What is screening?
A process which sorts out apparently well people who probably have a disease (or precursors or susceptibility to a disease) from those who probably do not
What are the types of screening?
Population-based screening programmes Opportunistic screening Screening for communicable diseases Pre-employment and occupational medicals Commercially provided screening
What is sensitivity?
The proportion of people with the disease who are correctly identified by the screening test
What is specificity?
The proportion of people without the disease who are
correctly excluded by the screening test
What is the positive predictive value?
The proportion of people with a positive test
result who actually have the disease
What is the negative predictive value?
The proportion of people with a negative
test result who do not have the disease
Why are positive predictive values so different?
Predictive values are dependent on underlying prevalence
What is lead time bias?
The length of time between the detection of a disease (usually based on new, experimental criteria) and its usual clinical presentation and diagnosis (based on traditional criteria).
What is length time bias?
Length time bias is a form of selection bias, a statistical distortion of results that can lead to incorrect conclusions about the data. Length time bias can occur when the lengths of intervals are analysed by selecting intervals that occupy randomly chosen points in time or space. That process favors longer intervals and so skews the data.
