GATE uses a triangle, circle, square and arrows to graphically represent the design of all epidemiological studies, including experimental (mainly randomised trials) and non-experimental (i.e. observational) studies
Every epidemiological study can be hung on the GATE frame, which was developed so users could visualise key epidemiological principles, study design features, and analytical and appraisal concepts
GATE illustrates how every epidemiological study shares the same basic structure and how every epidemiological study is designed with the same objective - to measure how much dis-ease occurs in different groups (or populations)
Dis-ease
Any health-related 'event' (e.g. an injury, a heart attack, a death) or health-related 'state' (e.g. diabetes, a disability, a raised blood pressure, or quality of life)
Epidemiologists generally study negative events or states, like death and disease, because they are easier to measure than positive states of health such as degrees of wellbeing
Occurrence (of dis-ease)
The transition from a 'non-dis-eased state' to a 'dis-eased state'
Population
Any group of people who share a specified common factor
Categorical data
Data grouped into categories (e.g. male/female, smokers/ non-smokers, dead/alive)
Numerical data
Data that take on numerical values (e.g. body weight, blood cholesterol levels, number of hospital visits)
Measuring dis-ease occurrence in a population can inform health service planners about types of health services required for populations, including health promotion, dis-ease prevention, disease diagnosis and treatment
Measuring dis-ease occurrence in different groups (e.g. smokers / non-smokers or Māori / non-Māori) in a population can identify risk factors, predictors & causes of dis-ease occurrence
Measuring dis-ease occurrence in groups treated with, say, different drugs or surgical interventions can help determine which treatments work best
Numerator
The number of people in whom dis-ease occurs
Denominator
The number of people in the study population
A key requirement of epidemiological studies is that the dis-ease outcomes counted (numerator) must all come from a defined population (denominator)
Epidemiology is the study of the occurrence of dis-ease in populations
GATE frame
Illustrates the component parts of all epidemiological studies: the triangle represents the Participant (or Study) Population, the circle represents the study-specific sub-Denominators (Groups), the square represents the Numerators (or Dis-ease Outcomes), and the arrows represent Time
Exposure Group Occurrence (EGO)
The occurrence of dis-ease in the Exposure Group
Comparison Group Occurrence (CGO)
The occurrence of dis-ease in the Comparison Group
Numerical measures are often converted into categorical measures
When numerical outcomes are not converted into categories, the average or mean outcome can be calculated in each group
Incidence
A measure of the occurrence of new cases of dis-ease in a population over a specified period of time
Prevalence
A measure of the proportion of a population that has a dis-ease at a given point in time
Comparison group (CG)
The group with low salt intake
Ideal study using categorical data
Everyone in P (the triangle) would also be in either EG or CG (the circle)
Everyone would be classified as either having a dis-ease outcome (a or b) or not having dis-ease (c or d) (the square)
The number of people in P should equal the number of people in EG & CG
The number of people in EG should equal the number of people in a & c
The number of people in CG should equal the number of people in b & d
Numerical exposure measures
Salt intake classified numerically
Numerical outcome measure
Blood pressure levels
Correlation coefficient
Measure of the association between numerical exposures and numerical outcomes
Incidence
Counting the number of onsets of dis-ease (events) occurring during a period of time
Prevalence
Counting the number of people with dis-ease at a point of time
Incidence is the most appropriate measure for dis-eases with an easily observable onset
e.g. number of hospital admissions for heart attacks over 5 years in smokers vs non-smokers
Incidence measures require the dis-ease outcome to be a categorical (e.g. yes / no) variable
Incidence Proportion (Risk)
Counts everyone who started the study in the denominator and everyone who has a dis-ease onset during the study time period in the numerator
Incidence Rate
A more exact measure of incidence, where once a person has an event, they are removed from the denominator. Participants are counted in units of person-time.
Unless the study has very long follow-up, high loss to follow-up, or high event rate, there will be little difference between Incidence Rate and Incidence Proportion
Incidence is typically calculated separately in the Exposure and Comparison Groups
Incidence measures always include a unit of time in the description
Prevalence
Counting the number of people with dis-ease at a point of time (Numerator) and dividing by the number of people in the study group at that point in time (Denominator)
Prevalence is the measure used when the transition from non-dis-ease to dis-ease state cannot easily be observed and counted