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differentiate between a suspected, probable and confirmed case of an infectious disease (e.g. a measles outbreak)

this will be defined by the case definition.

suspected - clinical features (e.g. fever and rash)

probable - clinical features + contact with confirmed case

confirmed case - clinical features and positive microbiology, serology etc.


what is a case definition?

set of standard criteria for deciding whether or not a person has a particular disease or health related event


define prevalence

a measure of the proportion of the population that has a given disease, condition or characteristic at a given time (or time period)


define point prevalence

no. cases at a point in time, compared with the total population


define period prevalence

no. cases identified over a period of time, compared with no. people in the population over this time period.

NB - not NEW cases (that's incidence!), just existing cases


define incidence

frequency of NEW cases in a defined population in a specified time period


define cumulative incidence (risk)

no. NEW cases occurring over a given period of time in the population at risk at the BEGINNING of the time period


what 4 different relative measures come under the term "relative risk"?

prevalence ratio
risk ratio
rate ratio
odds ratio


what does relative risk measure?

measures the strength of association between exposure and disease


what are the 4 different measures of impact of a risk factor?

- attributable risk (aka excess risk)
- attributable risk fraction (or %)
- population attributable risk
- population attributable risk fraction (or %)


what is attributable risk?

the excess incidence of our outcome that can be attributed to the exposure


what does using an attributable risk fraction adjust for?

the fact that the exposed group would have had some disease anyway - AR fails to take into account the underlying, background rate.

i.e. - not all illness, even in the exposed group, will be due to the exposure


what does the attributable risk fraction tell us?

what proportion of disease IN THE EXPOSED GROUP is attributable to the exposure


what does population attributable fraction tell us?

what proportion of disease in the POPULATION that is attributable to the exposure

e.g. interpret PAF of 0.96 as "96% of (outcome) in the population are attributable to (exposure)


what is a cross-sectional study?

a study in which data are collected on each study participant at a single point in time


aka prevalence study


what are the two types of cross-sectional study?

descriptive and analytical


what do descriptive cross-sectional studies do?

what do they measure?

collect info on frequency and distribution of health-related exposures or outcomes, in a defined population.

measure point or period prevalence of the outcome OR exposure


how are data typically collected for a cross-sectional study?



what do analytical cross sectional studies do?

investigate the association between exposure to risk factors and the outcome of interest

(NB - the info is collected simultaneously on each individual - no temporality)


what are the differences between an analytical and a descriptive cross-sectional study?

descriptive cross-sectional studies basically just find the prevalence of an exposure or outcome, whereas analytical cross-sectional studies look at both exposures and outcomes to investigate the association between the two


what types of bias are cross-sectional studies particularly susceptible to?

recall bias - if asked about exposures that occurred a long time ago

non-response bias - always an issue with surveys - look at what the response rate is in the study


what measures are used in analysis of a cross-sectional study?

prevalence (of disease OR exposure)
prevalence ratio and prevalence odds ratios - for outcomes or exposures


list some advantages of a cross-sectional study

- quick, cheap and easy (ish!)
- provides prevalence of risk factors and disease in a defined population
- useful for health service planning
- repeated studies can monitor changes over time


list some disadvantages of a cross-sectional study

- exposure and disease info collected simultaneously = problems with temporal sequence - disease may modify exposure etc
- studying prevalent cases = can miss out cases with quick recovery, or short survival
- bias - recall, non-response
- not useful for rare conditions


what is an ecological study?

a study carried out at the population (or group) level rather than at the individual level


what is a multi-group ecological study?

aka ecological correlation study.
compares different groups (or areas) at a point in time


what is a time-trend study?

a type of ecological study, aka a time series study.

examines data in a population over time.
investigates if changes in incidence correlate with changes in exposures over time.

can be long (e.g. seasonal variation) or short (e.g. daily variation)


give examples of information that might be available at a population level, but not an individual level?

to study these, we do an ecological study

GDP and other national statistics
per-capita consumption
etc. etc.


give some reasons to study groups/populations
(ecological studies)

- to investigate differences between populations
- to study group-level effects (e.g. seat belt law only works at a group level!)
- convenience and availability of group level data (e.g. air pollution data is only available at a group level)
- quick and cheap study design!


give 4 reasons why ecological studies must be interpreted with caution

1. confounders - often, you can't adjust for these due to lack of data
2. bias - data may be collected using different methods or definitions over time or in different places
3. ecological fallacy
4. migration of populations between groups can dilute differences