Frequency of Clinical Events Flashcards
Disease control levels
Primary care
- actions to prevent disease
Secondary care
- actions to detect disease early, with intention to reduce impact
Tertiary care
- actions to extend/improve life after diagnosis
Return on dollars invested is highest with ______ and lowest with _______
Primary care; tertiary care
Generalized disease pathway
Induction - exposure, interaction, undetectable Incubation/latent period - local onset, detectability Signs - systemic affect, clinical manifestation, tissue destruction
True experiments
Subjects are randomized to treatment, and receive specific treatments
- randomization and control
Quasi-experiments
Like a true experiment except no randomization
- control without randomization
Observational studies
Neither randomization nor control
- subjects self select their treatment
Ecologic observational studies
Ecologic, heterodemic, community level studies
- entire farms, communities, shelters, families are the subject
Measures of disease frequency
Rate versus risk
Measures of association
Assessing risk factor (disease)
- age, breed, sex, production cycle
- relative risk, odds ratio, incidence density ratio
Measures of disease impact
Attributable risk, attributable difference, population attributable risk
- assess impact of exposure in the population
Relative frequency
Use proportions, ratios, rates
Incidence
Expression of the force of disease
- applied to new cases in a period
- indicates movement from well to diseased
- a person can only be an incident case once!
2 ways to express incidence
- incidence rate (incidence density)
- cumulative incidence (risk, incidence proportion, attack rate)
Rate
An expression to describe a change in one quantity with respect to another quantity with the denominator featuring a time component
- denominator is in person/animal-time
Incidence rate formula
of new cases over a time period / sum (over all individuals) of the length of time at risk of developing disease
How long do you count the time for incidence rate?
Count time until:
- animal gets disease
- death from another case
- removed from herd
- study terminates
- animal undergoes intervention to render it non-susceptible
Simple cumulative incidence
Proportion of non-diseased individuals at the beginning of a period of study that become diseased during the period
- individuals must start out non-diseased, in order to be at risk for new disease
- at risk means they can get the condition
Cumulative incidence formula
of new cases over a time period / # of healthy animals at beginning of time period
Attack rate
Cumulative incidence rate for an outbreak
- numerator: # of new cases
- denominator: # of individuals exposed at the start of an outbreak or for a limited time
When is attack rate appropriate to use?
When exposure occurs in a very short and defined period of time
- affects very specific and defined populations
- ex: food poisioning, neonatal period, acute exposure to radiation
Cumulative incidence does not account for _______
Animals that fall out of observation
- adjust with actuary table/measurement to account for periods with no data for a specific animal
Incidence rate summary
- precise expression of disease force
- applicable to a group
- 100 animal-years can be accumulated several ways
- hard to interpret, less often used than CI
Cumulative risk summary
- very easy to interpret
- applicable to group or individual
- problematic when animals are lost in the time window
- similar to IR, reflects an overall average, disease force in the time window
Measures of frequency
- levels of control (primary, secondary, tertiary)
- incidence (defined!)
- rate (incidence density)
- risk (cumulative incidence_