lecture_06

lecture_06 - Two-Way Contingency Tables Lecture 06 ILRST-...

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    Two-Way Contingency Tables Lecture 06 ILRST- 411
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             Relative risks and odds ratios              What’s the differences? Probability is the likelihood of an event in relation to all  possible events. If a horse wins 2 out of every 5 races. Its  probability of winning is 2/5 (40%). Relative risk is defined as a ratio of probabilities. It  compares the incidence or risk of an event  among those  with specific exposure with those who were not exposed.
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    What’s the difference between RR and  OR                         MI                       Yes     No Smoking          355     3140 Non-smoking    140     2507 For example: the probability of having MI among the smokers is  compared to the probability of having MI among the non-smokers.     
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    What’s the difference between RR and  OR RR is based upon the incidence of an event given that we  already know the study participants’ exposure status. It is  only appropriate, therefore, to use RR for prospective cohort  studies. Relative risk = Incidence of disease among the exposed  /incidence of disease among the non-exposed.
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    What’s the difference between RR and  OR Odds compare events with non-events. If a horse wins 2 out  of every 5 races, its odds of winning are 2 to 3 (expressed  as 2:3). An odds ratio is a ratio of ratios. It compares the presence to  absence of an exposure given that we already know a  specific outcome. For example, a person having MI, the odds of being exposed  is compared with a person not having MI, the odds of being  exposed.
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    What’s the difference between RR and  OR Odds 1 = given that the person has the disease, the odds of  being exposed Odds 2 = given that the person does not have the disease,  the odds of being exposed Odds ratio=  Odds1/Odds2 OR can be used to describe the results of case control as  well as prospective cohort studies.
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             Comparing RR and OR OR and RR are usually comparable in magnitude when the  disease studied is rare (eg. Most cancers). However, an OR can overestimate and magnify risk,  especially when the disease is common (eg, hypertension)  and should be avoided is such cases if RR can be used.
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                  Odds ratio Odds are simply a ratio of the probability that an event will  occur versus the probability that an event will not occur, or  probability/1-probability For example, if you go fishing and you catch 3 largemouth  bass and 1 trout, then the odds of catching a trout = 
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This note was uploaded on 09/28/2007 for the course BTRY 6030 taught by Professor Das,t. during the Spring '06 term at Cornell.

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lecture_06 - Two-Way Contingency Tables Lecture 06 ILRST-...

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