Significance of odds ratio measures

Generally speaking OR (and Relative Risk see below) values greater than 1 indicates that a disease is more likely to occur in an exposed group as compared to an unexposed group. Conversely, an OR value less than 1 means that a disease event is less likely to occur in an exposed groups compared to unexposed group. An OR that has a 95% CI that overlaps with 1 is indicative of an OR that is not a significant (Box 5).

 Box 5.

Between 1996 and 2007 the apiary inspection programme in the Commonwealth of Pennsylvania inspected 19,933 apiaries for clinical signs of chalkbrood and sacbrood disease. Over all inspections, 1,831 apiaries were found to have at least one colony with chalkbrood, and 547 colonies were found to have sacbrood. 212 apiaries had colonies infected with chalkbrood and sacbrood at the same time (vanEngelsdorp, unpublished data). Was there an association between the presence of chalkbrood and sacbrood?

Box 5 Table



Thus, apiaries infected with chalkbrood are 6.9 times more likely to be infected with sacbrood when compared to apiaries not infected with chalkbrood.

Box 5 Equation 2

Thus, the 95 % confidence interval in this example is = 5.8 -8.3.  The confidence interval does not include 1.0, therefore the relationship between Sacbrood and Chalkbrood is statistically significant, and is unlikely due to chance.