Definitions Flashcards
Exposure
Any factor that may influence the outcome e.g. smoking
Outcome
The disease, or event, or health related state under study e.g. lung cancer
Observational epidemiology
Describe patterns of health and disease without intervening to change the factors which influence them
Interventional Epidemiology
Assess the effect of a specific intervention; individual level or community level
Prevalence
The number of existing cases in a population at a designated time
Point prevalence
The proportion of persons in a defined population that has the outcome under study at a specific point in time
Period prevalence
The proportion of persons in a defined population that has the outcome under study over a period of time
Incidence
The number of new cases in a defined time
Population pyramid
A graphic representation of the age and sex composition of the population
Spike pyramid
High birth rate, high death rate, low growth rate (in equilibrium)
Wedge pyramid
High birth rate, low death rate, high growth rate (in transition)
Barrel/beehive pyramid
Low birth rate, low death rate, low growth rate (in equilibrum)
Dependency ratio
the proportion of economically inactive people within the population (due to age - under 15s and over 65s)
Infant mortality rate
The number of deaths in infants aged under 1 year per 1000 live births; measure of a populations state of health and quality of healthcare
Maternal mortality ratio
The ratio of the number of maternal deaths during a given time period per 100,000 live births during the same time period
Fertility rate
The number of live births per 1000 women per year
Total period fertility rate
The average number of children that would be born to a women over her lifetime. TPFR >2.1 = increasing population; TPFR <2.1 = decreasing population
Life expectancy
The number of years a baby born today can be expected to live if it experienced the current age specific mortality rates
Healthy life expectancy
Expected years of life in good or fairly good general health
Disability free life expectancy
Expected years of life without a long term limiting illness
Potential years of life lost
A measure of the relative impact of various disease and lethal forces on society (highlights the loss to society form early deaths)
Disability adjusted life years
Measure of burden of disease (the extent to which a disease reduces healthy years if life); 1 DALY = 1 healthy year of life lost
Cross sectional study
Estimate frequency or outcome by comparing different groups at the same point in time
Random sampling
Each subject has equal chance
Stratified sampling
Population divided into groups (e.g. income or age), take a random sample from each group
Ecological study
Observational study with populations or groups (not individuals); compares group averages
Ecological fallacy
An attempt to infer from the ecological level to the individual level
Family aggregate analysis
Are relatives of a person with the disease more likely to have the disease than the general population
Twin studies
Quantifies the relative contributions of genetic and environmental factors to a disease; compares the concordance rate of disease in monozygotic twins to dizygotic twins
Adoption studies
Compare disease concordance with biological parents to disease concordance with adoptive parents
Segregation analysis
Analyses the mode of inheritance of a disease and how many genes are involved; involves studying pedigrees to see if inheritance follows a pattern
Linkage studies
A method for determining the region of genome that contains a disease susceptibility locus using genetic markers
Association studies
The tendency for 2 or more characters to occur together at a frequency more or less than would be expected from their individual frequencies
Genetic markers
Single nucleotide polymorphisms (SNPs), copy number variation (CNV), insertion/deletion (INDEL)
Family based studies
Involves affected individuals and their parents
Genome wide association studies (GWAS)
Genetic markers are analysed covering the whole genome
Linkage disequilibrium
Non-random correlation of alleles at 2 different loci; their occurence together is observed more or less often than expected from their individual frequencies
Relative risk / risk ratio (RR)
Assess strength of association - compares incidence of disease between those exposed and those unexposed
Risk difference (RD)
Measures clinical and public health importance of the causal relationship
Person years
Total number of years the study subjects have contributed
Temporality
Exposure status is defined before outcome is assessed (ie. the time sequence of events can be established)
Prospective study
Measures risk factors ‘today’ then follows up into the future
Retrospective study
Outcome of interest is examined ‘today’ and then history of exposure is determined from health, civic records etc
Cases
Have the disease of interest
Controls
Do not have the disease of interest
Odds ratio
How likely or not an individual is to develop a disease if they are exposed
Missclassification bias
True cases are wrongly labelled as controls (or vice versa)
Observer bias
Knowledge of case/control status may influence data collection
Survivor bias
Occurs if exposure is rapidly fatal (pt dies before they become known as a case)
Confounding
A confounder is a variable that is independently associated with exposure and outcome
Routine data
Data that is collected routinely in a standardised and consistent way
Standardisation
Often used to control for confounding effects of age so that rates of disease or mortality can be compared in populations with different age structures
Direct standardisation
Age specific rates from a population of interest are applied to a standard population
Indirect standardisation
Finds the number of deaths expected if both populations had the same (standard) age specific death rates, but kept their real age structure
Standard mortality ratio (SMR)
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
Bias
Systematic deviation from the truth
Selection bias
Systematic differences in characteristics between those who take part in a study and those who do not (or those in different groups)
Information bias
Any error in the measurement of exposure or outcome that results in systemic differences in the accuracy of data collected between comparison groups
Recall bias
Those with outcome report exposure with greater accuracy / minimise or conceal incriminating evidence
Observer/interviewer bias
when the accuracy of exposure or outcome data recorded by the investigator differs systematically between subjects in different exposure groups
Differential bias
The likelihood of misclassification of exposure is associated with outcome (and vice versa)
Non-differential bias
Equal amount of misclassification has occurred in the same direction
Effect measure modifier
Occurs when the magnitude of the effect of the primary exposure on an outcome differs depending on the level of a third variable
Chance
Influences the results of all studies, by varying amounts
Attributable risk (AR)
The difference between the incidence in exposed and the unexposed; it is the incidence of the disease (among exposed individuals) caused by the exposure
Population attributable risk (PAR)
The incidence of disease in a population attributable to the risk factor; the absolute difference between risk in the total population and the unexposed population
Attributable risk percentage (AR%)
The proportion of disease among the exposed which can be attributed to eh exposure and could be avoided by eliminating the exposure
Population attributable risk percentage (PAR%)
Proportion of cases in the population attributable to the exposure
Causation
Implies that there is a true mechanism that leads from exposure to disease
Association
An identifiable relationship between an exposure and disease
Cause
An event, condition, or characteristic without which the disease would not have occurred
Necessary cause
An exposure which is necessary for disease to occur
Sufficient cause
A set of conditions is a sufficient cause when it always produces the outcome; requires the joint action of many component factors
Efficacy
Can this intervention work under ideal conditions
Effectiveness
Does this intervention work under normal conditions
Explanatory studies
Show if something can work under ideal conditions
Pragmatic trials
Show if something does work in practice
Comparator
Can be a placebo, alternative therapy or usual care
Allocation concealment
The allocation sequence is not known to anybody involved in enrolling participants in the study
Block randomisation
Ensures that the number of participants allocated ti each group is equal after every block of X patients have entered the study
Stratified randomisation
Useful if other risk factors have a strong influence on the outcome
Minimisation
Aims to balance treatment allocation across multiple factors (age, sex, disease severity etc); similar to stratified randomisation
Crossover study design
Each patient gets both treatments, with half receiving A first and half receiving B first
Cluster randomisation
When people are organised in natural groups, clusters are randomised not individuals
Stepped wedge study design
Randomise groups into a sequence of treatment, all groups receive intervention but not all at the same time
Intention to treat analysis
Compares outcomes for all randomised individuals - even if they stop taking treatment of drop out of the study; assesses the overall effect of assigning a subject t receive a particular intervention
Per protocol analysis
Includes only those who finished the trial and took the drugs/underwent the intervention; may introduce bias due to selective drop out
Performer bias
systematic differences in the care provided to the participants in the comparison groups other than the intervention under investigation
