CH. 12 - relative risk (RR/OR)

differences b/w matched and unmatched:

"unmatched" has "cases & controls" in different places

unmatched odds ratio for case-control study

*ad / bc*

odds ration in matched-pairs case-control study

*concordant pairs* • pairs that had the same exposure experience *discordant pairs* • pairs that had different exposure experience

calculations

1. disease risk in exposed / disease risk in nonexposed 2. disease risk in exposed - disease risk in nonexposed

odds ratio in case-control studies (2 types):

1. unmatched 2. matched

RR!

ONLY has whole #'s (no odd numbers b/c it must be even when we divide)

AR is important, why?

comparison is fundamental in epidemiology -- allows for the forming of relationships

similarities b/w matched and unmatched:

in both, letters are in the same order & exposure/ no exposure is placed on the left

odds ration (relative odds)

in order to calculate a relative risk, we must have values for the incidence of the disease in the exposed & the incidence of the disease in the unexposed

for a case-control study calculate...

odds ratio (OR)

for a cohort study calculate...

relative risk (RR)

interpretation of odds ratio

same as relative risk • if exposure is NO related to disease development then it is 1 • is exposure is positively related to disease then it is greater than 1 • is exposure is negatively related to disease then it is less than 1

determining excess risk

• calculate the ratio of attack rates in those who ate the food to those who did NOT eat the food • subtract the risk in those who ate the food and those who did NOT eat the food -- the difference represents the excess risk in those who were exposed

case-control and odds ratio (OR)

• cannot calculate the relative risk directly to determine whether there is an association b/w the exposure and the disease • why? we do NOT have the information about the incidence of disease since we already started with cases and control and NOT exposure • then what? - we use odds ratio as a measure of association b/w exposure and disease - ratio of the odds that the cases were exposed to the odds that the controls were expected

concordant pairs

• case & controls exposed "a" • neither case nor control exposed "d"

disconcordant pairs

• case & controls not exposed "b" • control exposed and case not exposed "c"

to answer whether there is an association b/w the exposure and the disease, we can either use the relative risk o we can use odds ration (relative odds)

• defined as the ratio of the odds development of the disease in exposed persons to the odds of development of disease in nonexposed persons *ad/bc

interpreting relative risk

• if *RR=1* -- risk in exposed is EQUAL to risk in nonexposed (no association) • if *RR>1* -- risk in exposed is GREATER than risk in nonexposed (positive association; possibly causal) • if *RR<1* -- risk in exposed is LESS than risk in nonexposed (negative association; possibly protective)

calculation of odds ratio in matched-pairs

• ignore concordant pairs • use only the discordant pairs • odds ratio (matched pairs) = *b/c* (a/d = measuring same thing, essentially so they cancel out! *so, we only use b & c , because they are different*

implications of absolute risk

• implications in both clinical and public health policy (example. a women who contracts rubella in the 1st trimester of pregnancy and asks her doctor " what is the risk that my child will be malformed"? -- go get info. from past cases)

OR as a good estimate of RR

• in a case-control study, ONLY the OR can be calculated as a measure of association (NOT RR) • in a cohort study, OR & RR can be calculated as a measure of association

calculating relative risk (RR)

• in a cohort study the RR can be calculated *DIRECTLY* *a / a+b* = incidence in exposed individuals *c / c+d* = incidence in nonexposed individuals *RR= Ie / Io

absolute risk

• incidence of a disease in a population •indicates the *magnitude of the risk in a group of people* with a certain exposure • does NOT take into consideration the risk of disease in nonexposed individuals • it does NOT indicated whether the exposure is associated w/ increased risk of the disease

odds ratio (OR)

• measure of association • can be obtained in a case-control and a cohort study • can be used INSTEAD of the relative risk

determining assocoation

• must do *case-control* OR *cohort study* • ex. foodborne outbreaks • determine attack rate of those exposed and nonexposed

conclusion...

• odds ratio is the same for both cohort and case-control studies • 3 steps: - multiple ACROSS 2X2 table (2 steps) • divide

relative risk (RR)

• risk in exposed/ risk in nonexposed • tells you how strong an association is b/w an exposure and the development of a disease

case-control study

• we do NOT know the incidence in the nonexposed population b/c we start with diseased people (cases) and nondiseased people (controls) • therefore, *we can NOT calculate the relative risk in a case-control study* • can, however, obtain a *good estimate of the relative risk* in a control study

odds ratio (OR) in cohort study

• what is the probability (P) that the disease will develop in an exposed person? * P = a / a+b * *OR = a / b * • what is the probability (P) that the disease will develop in an nonexposed person? * P = c / c+d * *OR = c / d *