Attack Rates and Ratios, Exercises of Social Psychology

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Attack Rates and Ratios^ Sharon L.R. Kardia

Objectives
^ To quantify the proportion of people in a communitywho are affected by an epidemic or outbreak
^ To investigate potential causes of an outbreak using theattack rate statistic
^ To evaluate hypotheses about what caused an outbreakusing attack rate ratiosKey Words
^ Crude attack rates, food-specific attack rates, attackrate ratios^

Identify and quantify cases and exposed
Components of case definition^ ^ Person^
^ Clinical information (symptoms and/or lab)^
^ Characteristics of the affected population^ ^ Place^
^ Information about the location^ ^ Time^
^ Period of the outbreak

Attack Rates (AR) are used to evaluatethe outbreak^ Attack Rate (AR) =Number of people at risk who develop a certain illness^ Total number of people at riskOften presented as a % (percent)^
^ Can be calculated for everyone at risk (“Crude AttackRate”)^
^ Can be calculated for people who had a specificexposure (“Food-Specific Attack Rate”)^
^ Can be compared to the Attack Rate in people who didNOT have the exposure^
^ Could be compared to endemic frequency of thedisease

Food-Specific Attack Rates

People who ate Item Calculate Risk Specific Attack Rate^ Which item shows the highest food-specific attack rate?Is the attack rate low among those who did not eat the item? People who did not eat item Ill^ Well^ TotalItem AR(%)^ Ill^ Well^ Total^

AR(%)

(^29 17 46) Baked Ham

(^26 17 43) Spinach

(^23 14 37) Mashedpotatoes

Cabbage^18 10 28 salad

(^16 7 23) Jell-O

(^21 16 37) Rolls

(^2 2 4) Milk

(^43 11 54) Cantaloupe

How are food-specific attack ratesinterpreted?^ ^ Using the statistics for ice cream:^
^ Of the people attending the July picnic in Lansing andwho ate cantaloupe, 80% became ill with diarrhea^
^ Of the people attending the July picnic in Lansing andwho did not eat canteloupe, 14% became ill with diarrhea

Evaluate hypotheses using Attack RateRatios

# people who ate Item^ # people who didnot eat itemIll^ Well^ AR(%)^ Ill^ WellItem

AR(%)^ ARR Baked^29 17 63 Ham

17 12 59 (^26 17 60) Spinach 20 12 63 (^23 14 62) Mashedpotatoes 23 14 62 Cabbage^18 10 64 salad

28 19 60 (^16 7 70) Jell-O 30 22 58 (^21 16 57) Rolls 25 13 66 (^2 2 50) Milk 44 27 62 (^43 11 80) Cantaloupe 3 18 14

Interpreting the Attack Rate Ratio^ ^ The ratio of those who ate cantaloupe andbecame ill to those who did not eat thecantaloupe and became ill was 5.71^ ^ Individuals who ate cantaloupe were 5.71times more likely to become ill than thosewho did not eat cantaloupe at the Julypicnic.^ ^ Cantaloupe is the likely source of illnessin this outbreak^

(^11) Wikipedia commons