Measures of disease frequency/association

Question 1

Incidence risk (R) - definition, characteristics, approximation

Answer 1

= Cumulative incidence. Probability that an animal will contract the disease in a defined time period.

• Should be restricted to a closed population where animal is observed for full risk period.
• No units (but should define time period)
• Ranges from 0 to 1.
• Denominator (population at risk) - only disease free animals at beginning should be monitored.
• Bias: Estimate can be biased if animals lost to follow up (withdrawals) - if few animals are lost then bias is small. Can be handled by subtracting half of animals that withdraw from the denominator (assumes on average animals leave half way through period of observation)

Question 2

Incidence rate (I) - definition, characteristics, approximation

Answer 2

= Incidence density. Number of new case per animal-unit of time.

• Units are per unit of animal-time.
• Positive, no upper limit
• Denominator (# animal-time units at risk) can be estimated as [(# beginning - 1/2 # diseased - 1/2 # withdraw + 1/2 added)*time] where time is the length of the study period and is the same for all animals

Question 3

Prevalence - definition

Answer 3

Proportion of population affected by disease at a specific point in time. Equivalent to the probability of randomly selecting an animal with the disease.

Question 4

Attack rate - definition

Answer 4

Equivalent to an incidence risk in an outbreak setting

• Used when risk period is limited and all cases arising from the exposure are likely to occur within the risk period

Question 5

Standardization - application, methods (2)

Answer 5

Used when comparing risk/rates across populations when the distribution of host characteristics differs between populations and may explain the observed difference in disease rates (i.e. confounding). Standardized values represent a weighted average

Methods:

1. Direct standardization: calculate age-specific rates, multiply rate by number of animals in reference population to yield expected cases, calculate age-adjusted rate by summing up all expected counts and dividing by total population (alternatively can just multiply age-specific rates by % in reference population and sum the values)
2. Indirect standardization: same age-specific rates are applied to population A and B (i.e. multiply rate by number of animals in each study population to yield expected cases in each group), sum up expected cases and compute SMR (observed vs expected); standardized rate is rate in standard population multiplied by SMR

Question 6

Attributable risk - formula, interpretation, applicable study design

Answer 6

I_exposed - I_unexposed

Excess risk of disease in the exposed, due to exposure. In animals that are exposed, there is an excess risk of XX due to exposure. Attributable risk of zero means exposure has no effect.; negative (protective); positive (positively associated with disease). Can calculate in cohort studies and cross-sectional studies (when prevalence is a good measure of risk).

Question 7

Attributable fraction - formulas (2), interpretation, applicable study designs

Answer 7

I_exposed - I_unexposed /

I_exposed

alternative: (RR - 1)/RR

Proportion of disease in exposed individuals that is due to the exposure. Can be calculated in cross-sectional, case-control studies (when OR is good approximation of RR) and cohort studies.

Question 8

Population attributable risk - formulas (2), interpretation, applicable study design

Answer 8

I_population - I_unexposed

where I_population can be estimated as incidence of disease in study population

AR*p(exposure)

Increase in risk of disease in the entire population that is attributable to the exposure. Dependent on stength of association and prevalence of exposure. Can be calculated in cross-sectional and cohort studies.

Question 9

Population attributable fraction - formulas (3), interpretation, applicable study designs

Answer 9

I_population - I_unexposed/

I_population

where I_population can be estimated as incidence of disease in study population.

P_exposure*(RR-1)/

P_exposure(RR-1) + 1

Alternatively: PAR/P_disease

Proportion of disease in the entire population that is attributable to the exposure. Dependent on stength of association and prevalence of exposure. Can be calculated in cross-sectional, case-control and cohort studies.