Comparing the occurrence of a characteristic in two groups of different sizes. Something occurs with one frequency in one group and with a different frequency in another group. To determine in which group it occurs proportionally more or less frequently, the ratios of occurrence and non-occurrence have to be compared, so these have to be put into relation. Mathematically, this means a/b divided by c/d, which leads to the following formula:
R = ad / (bc)
The odds ratio compares the odds, if a property in two different groups is present or not. At 1, the odds are identical, if less than 1, the odds are bigger for the first group, if more than 1, the odds are bigger for the second group. The odds ratio is used e.g. in medicine to determine risk factors.
Please fill in two of the three fields for each group. The third value and the odds ratio will be calculated. The total size is the size with the characteristic plus the size without the characteristic.
An example with invented numbers: women (group 1) and men (group 2) are checked for a certain disease. This occurs at 43 women (a=43) and doesn't at 3734 (b=3734). In contrast, 84 men (c=84) are affected, 3498 aren't (d=3498). The ratio is 0.48, which gives men a more than doubled chance to get this disease. The fact that slightly more women, namely 3,777, were tested than men (here it was 3,582), is irrelevant for the odds ratio, but this value can still contain important information. If the group sizes are very different or both are very small, the measurement is probably not particularly representative.
To avoid distortions caused by other effects, both groups should be as similar as possible except for the distinguishing property.
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See also Statistics Calculator