In a 2by2 table with cells a, b, c, and d (see figure), the odds ratio is odds of the event in the exposure group (a/b) divided by the odds of the event in the control or nonexposure group (c/d). Thus the odds ratio is (a/b) / (c/d) which simplifies to ad/bc.
What is the stratified CMH method?
In statistics, the Cochran–Mantel–Haenszel test (CMH) is a test used in the analysis of stratified or matched categorical data. It allows an investigator to test the association between a binary predictor or treatment and a binary outcome such as case or control status while taking into account the stratification.
What is the difference between odds ratio and crude odds ratio?
To briefly summarize: a crude odds ratio is just an odds ratio of one IV for predicting the DV. The adjusted odds ratio holds other relevant variables constant and provides the odds ratio for the potential variable of interest which is adjusted for the other IVs included in the model.
How do you calculate crude risk ratio?
With a cohort study the "crude" RR is estimated naturally by the ratio of the two observed rates, i.e., by f>e = (e/F)/(g/H) = eH/gF.
How do you manually calculate odds ratio?
So case control studies the measure of association that we would calculate is called an odds ratio odds ratios are just that a ratio of odds. So in this case will be the odds of being exposed to
What is a stratum specific odds ratio?
Stratumspecific ORs or RRs are calculated within each stratum of the confounding variable and compared with the corresponding effect estimates in the whole group (that is, with the unstratified OR or RR).
What is the odds ratio of no difference?
The odds ratio (OR) is the ratio of odds of an event in one group versus the odds of the event in the other group. An RR (or OR) of 1.0 indicates that there is no difference in risk (or odds) between the groups being compared.
Frequently Asked Questions
What does 1.4 odds ratio mean?
Odds Ratio is a measure of the strength of association with an exposure and an outcome. OR > 1 means greater odds of association with the exposure and outcome. OR = 1 means there is no association between exposure and outcome. OR < 1 means there is a lower odds of association between the exposure and outcome.
What is the meaning of crude odds ratio?
An odds ratio (sometimes called a “crude” odds ratio) is useful for telling us how changes in one predictor variable affect the odds of some response variable occurring.
What is the point of no difference odds ratio?
The odds ratio (OR) is the ratio of odds of an event in one group versus the odds of the event in the other group. An RR (or OR) of 1.0 indicates that there is no difference in risk (or odds) between the groups being compared.
How do you interpret CI for odds ratio?
The 95% confidence interval (CI) is used to estimate the precision of the OR. A large CI indicates a low level of precision of the OR, whereas a small CI indicates a higher precision of the OR. It is important to note however, that unlike the p value, the 95% CI does not report a measure's statistical significance.
What is the formula for the odds ratio for a matched casecontrol study?
In a 2by2 table with cells a, b, c, and d (see figure), the odds ratio is odds of the event in the exposure group (a/b) divided by the odds of the event in the control or nonexposure group (c/d). Thus the odds ratio is (a/b) / (c/d) which simplifies to ad/bc.
What is the matching ratio in casecontrol?
Because the number of cases (which are often rare diseases) is usually much smaller than that of potential controls, the matching ratio (i.e., ratio of cases:controls in each matched set) is often set to 1:n. If the ratio is set to 1:1, the design is called a pairmatched casecontrol study.
FAQ
 Can you use odds ratio in casecontrol?
 Many epidemiologists and statisticians believe that the odds ratio is the only measure that can be reliably estimated from casecontrol studies.
 How do you pool odds ratio?
 To calculate a weighted average, each individual value is multiplied by its weight and these new values are then added up and divided by the sum of the weights. Various sets of weights can be used for pooling odds ratios, but those proposed by Mantel and Haenszel (1959) are commonly used.
 What is the formula for pooled risk ratio?
 If the 'try exact' option is not selected then a normal approximation to the confidence interval is given instead. The MantelHaenszel type method of Rothman and Boice (Rothman, 1998) is used to estimate the pooled risk ratio for all strata under the assumption of a fixed effects model:  where ni = ai+bi+ci+di.
 What is the formula for calculating odds ratio?
 In a 2by2 table with cells a, b, c, and d (see figure), the odds ratio is odds of the event in the exposure group (a/b) divided by the odds of the event in the control or nonexposure group (c/d). Thus the odds ratio is (a/b) / (c/d) which simplifies to ad/bc.
 What is a pooled RR?
 The pooled odds ratio (OR) and risk ratio (RR) are measures of the effect size in a metaanalysis. They are calculated by combining the results of multiple studies and are used to estimate the overall effect of an intervention or exposure on an outcome.
 What is the odds ratio for dummies?
 The odds ratio is the ratio or comparison between two odds to see how they change given a different situation or condition. The odds ratio for a feature is a ratio of the odds of a bike trip exceeding 20 minutes in condition 1 compared with the odds of a bike trip exceeding 20 minutes in condition 2.
How to calculate crude odds ratio from stratified
What is the confounding variable for odds ratio?  To explore and adjust for confounding, we can use a stratified analysis in which we set up a series of twobytwo tables, one for each stratum (category) of the confounding variable. Having done that, we can compute a weighted average of the estimates of the risk ratios or odds ratios across the strata. 
How do you know if something is a confounding factor?  A variable must meet two conditions to be a confounder:

How do you identify a possible confounding variable?  A confounding variable is one that could potentially have an effect on both the independent and dependent variables in a study. In this case, it is possible that there is an association between smoking and caffeine as well as smoking and lung cancer. 
How do you know if confounding is present?  If the adjusted effect differs substantially from the crude effect, then confounding is considered to be present. In the example, the ageadjusted effect of diabetes mellitus will be 1.5, as coincidentally, both stratumspecific estimates are 1.5. 
What does odds ratio of 1.5 mean?  As an example, if the odds ratio is 1.5, the odds of disease after being exposed are 1.5 times greater than the odds of disease if you were not exposed another way to think of it is that there is a 50% increase in the odds of disease if you are exposed. 
 What is the effect modifier of the odds ratio?
 With effect modification, we expect the crude odds ratio to be between the estimates of the odds ratio for the stratumspecific estimates. Effect modifier is a variable that differentially (positively and negatively) modifies the observed effect of a risk factor on disease status.
 How do you find the effect modifier?
 To check for effect modification, conduct a stratified analysis. If the stratumspecific measures of association are different than each other and the crude lies between them, then it's likely that the variable in question is acting as an effect modifier.
 What is the odds ratio for confounders?
 The 10% Rule for Confounding The magnitude of confounding is the percent difference between the crude and adjusted measures of association, calculated as follows (for either a risk ratio or an odds ratio): If the % difference is 10% or greater, we conclude that there was confounding.
 What is the 10 percent rule for confounding?
 Identifying Confounding In other words, compute the measure of association both before and after adjusting for a potential confounding factor. If the difference between the two measures of association is 10% or more, then confounding was present. If it is less than 10%, then there was little, if any, confounding.
 How do you calculate the odds ratio?
 In a 2by2 table with cells a, b, c, and d (see figure), the odds ratio is odds of the event in the exposure group (a/b) divided by the odds of the event in the control or nonexposure group (c/d). Thus the odds ratio is (a/b) / (c/d) which simplifies to ad/bc.