Epidemiology Flashcards
what is prevalence
the number of existing cases in a population at a designated time
what is incidence
the number of new cases
relationship between prevalence and incidence
prevalence = incidence x average disease duration
name the three types of population pyramid
-spike - high birth rate, high death rate
-wedge - high birth rate, low death rate, high growth rate
-barrel - low birth rate, low death rate
what is infant mortality rate
no. of deaths of infants aged 0-1years/no. live births
what is a total period fertility rate (TPFR)
the average number of children that would be born to a woman over her lifetime
define life expectancy
the number of years a baby born today can be expected to live if it experienced the current age-specific mortality rates
what is healthy life expectancy
expected years of life in good or fairly good general health
what is the PYLL index
potential years of life lost
-a measure of the relative impact of various diseases and lethal forces on society
(the number of years of life lost when a person dies prematurely)
what is dependency ratio
a demographic measure of the ratio of the number of dependents to the total working age population in a country or region
how do you calculate dependency ratio
under 15 plus over 65 years/population ages 15-64 years
what’s a cross sectional study
estimate frequency or outcome at a particular point in time
-descriptive or analytical
why do a cross sectional study?
-health service planning - prevalence of specific outcome in a defined population at a point in time
-useful for assessing burden of disease and planning preventative and curative services
-not useful for rare diseases
-generate hypotheses about causes
(these studies prove association not causation)
survey sampling
-can make statements about the population by asking a (small) sample
-a well taken sample is (almost) as informative as a complete census
-sampling is a feature of all research designs
what is simple random sampling
-list the group
-generate random numbers
-contact selected individuals
-collect data
what bias in cross-sectional studies
-selection bias (characteristics of those taking part vs those not taking part)
-information bias (recall bias)
advantages of cross sectional
-easy and economical
-provides important information on the distribution and burden of exposures and outcomes - health service planning
-can be used as the first step in the study of a possible exposure-outcome relationship
weaknesses of cross sectional
-limited value for investigating aetiological relationships
-can be difficult to establish the time-sequence of events, the exposure may have occurred as a result of the outcome (reverse causality)
-not good for rare diseases
-selection bias (characteristics of those taking part/not taking part)
-recall bias
-could generate hypotheses about causes
what is an ecological study
observational study with populations or groups (instead of individuals) being unit of observation
what are the uses of an ecological study
-described association at group level
-quick and cheap - routine data
-generates hypotheses - first step
-some risk factors may not easily be measurable at an individual level
what is ecological fallacy
-ecological studies enable us to make ecological inferences about effects at the group-level
-they do not enable us to make inferences about individual risks
-an attempt to infer from the ecological level to the individual level is often called an ecological fallacy
purpose of randomised control trials
-minimise selection bias
-minimise confounding
-if participants are blind to treatment allocation then reporting bias is minimised
-if investigators are blind to allocation then observer bias is minimised
-provides powerful evidence of a causal relationship between the intervention and the outcome
what is incidence risk
the number of new cases in interval/population initially at risk
what is incidence rate
the number of new cases/total person-time at risk
what is incidence rate good for
dynamic populations
what is incidence risk good for
static populations
what is annual incidence
counts the deaths over a calendar year
what is cumulative incidence
frequency of new cases over a specified period
what to consider when deciding if risk factor and disease is causal (10 marks)
-consider relative risk, hazard ratio and odds ratio to determine strength of association
-attributable risk
-define causation and association
-consider that aetiology of chronic disease is often difficult to determine
-some exposures may cause more than one outcome
-outcomes may be due to multiple exposures or continual exposure over time
-causes may differ by individual
-probabilistic causation is when causal factor increases the chance of disease
-a sufficient cause is a complete causal mechanism that always produces disease
case control main features, advantages and disadvantages (10 marks)
-measure exposure in cases vs controls
-select (diseased) cases and (undiseased) controls
-use odds ratio to interpret results
-selection bias can occur as people who participate are only those who are eligible
-observer bias can occur if knowledge of case/control status
-same with participant bias
-there can be rare outcomes
-time varying exposures
-can assess multiple exposures
-there wouldn’t be any recall bias
-can prove a good causal relationship between exposure and outcome
advantages of case-control studies
-no recall bias
-can show rare outcomes (from rare diseases)
-can look at multiple exposures/risk factors simultaneously
-time varying exposures
-quick, inexpensive and easy
-useful initial studies to establish an association
ecological fallacy 6 marker
-can happen in ecological studies
-define ecological studies
-ecological fallacy occurs when a researcher attempts to infer from the ecological level to the individual level
-ecological studies do not enable us to make inferences about individual risks
-there are complex associations between disease and exposure
-countries with high or low exposure differ systematically in many other ways
what does it mean if p<0.05
reject chance
-conclude real effect
what does it mean if p>0.05
cannot exclude chance
-cannot conclude there is real effect
advantages of randomised control trials
-minimises selection bias and confounding
-if participant blind to allocation then minimises reporter bias
-if observer blind to allocation then minimises observer bias
-multiple outcomes can be examined
-incidence rate of outcome can be measured
disadvantages of randomised control trials
-very time consuming - trials can take years
-recruitment is difficult and time consuming
-impossible to do interventional trials sometimes
-expensive
advantages of cohort studies
-temporality - exposure status defined before outcome
-can measure multiple outcomes from rare exposures
-no recall bias
-its possible to estimate all measures of incidence and effect
disadvantages of cohort studies
-huge investment of time, human resources and financial resources
-reproducibility is hard
-loss to follow up - bias can be introduced which is difficult to control
-requires large sample size
-uncontrolled confounding variables
-inefficient for rare diseases
two types of information bias
reporter and observer
reporting bias
-when subjects give an answer they think will please the investigator
-when subjects conceal potentially embarrassing info
-when subject with a specific health outcome report previous exposure with a different degree of accuracy to other subjects
-when subject who have experienced a specific exposure report subsequent health events to a different degree of accuracy to others
how can you avoid/minimise reporting bias
-use exposure data before outcome
-objective data sources where possible
-keep subjects unaware of association under study
what is observer bias
-when accuracy of exposure data recorded by investigator differs (case control and cross sectional)
-when accuracy of outcome data differs (cohort and intervention studies)
-interviewer bias
how do you avoid/minimise observer bias?
-the people who are responsible for classifying the outcome do not know the subjects’ exposure category
-blinding
intention to treat
-compares outcomes for all randomised individuals (even if they stop taking treatment or drop out of study)
-assesses the overall effect of assigning a subject to receive a particular intervention
-analysis is the most important analysis as intervention and control groups compared as originally randomised
-more likely to underestimate treatment effect
(in randomised control trials lecture)
short notes on incidence risk and rate (6 marks)
-incidence measures number of new cases in a group/population
-incidence risk is number of new cases in an interval/population initially at risk
-incidence risk is good for static populations
-incidence rate is number of new cases /total person-time at risk
-incidence rate measures the rate at which a new disease occured over a period of time
-incidence rate is good for dynamic populations
-annual incidence counts deaths over a calendar year
-cumulative incidence is the frequency of new cases over a specified period
Cluster sampling
6 mark short notes
-cluster sampling involves dividing a population into clusters and then randomly selecting a sample of these clusters
-clusters are randomised
-there must be a lot of clusters to randomise to avoid unequal distribution
-need increased sample size of individuals
-subjects within a cluster may be different to subjects in other clusters so ideal is lots of small clusters
-analysis more complicated as need to take account of cluster design
-allocation concealment more difficult
sampling frame
6 mark short notes (only have 4 points rn)
-a sampling frame is a list of all the people forming a population from which a sample is taken
-simple random sampling is when people in the group are allocated random numbers
-the sampling frame comes from an initial group of interest from which individuals are selected
-convenience samples are taken from a place that is convenient for that study
SMR
short notes
-standard mortality ratio
-a measure, expressed as either a ratio or percentage to quantify an increase or decrease in mortality in a study cohort compared to the general population
-calculated from expected deaths in a study population
-compares observed deaths with expected deaths
-needs number of persons in each age group in population being studied
-age specific death rates of general population by same age groups
-observed deaths in study population
