Odds Ratios

Some studies, usually meta-analyses, give the chances of a certain event happening as an odds ratio.

This can make interpreting the results a little confusing, as obviously percentages vary from 1-100% and ratios from 0-1, but odds ratios can be any number from 0 to infinity.

There is an article about odds ratios and their use in medical studies on the Bandolier (Evidence-Based Healthcare Journal) website.

In brief, from that article, "If the odds of an event are greater than one the event is more likely to happen than not (the odds of an event that is certain to happen are infinite); if the odds are less than one the chances are that the event won't happen (the odds of an impossible event are zero)."

This is just like the odds offered by a bookmaker on, say, the outcome of a horse race. When an odds ratio is 0.5, that means the odds are 0.5:1 of the event happening. A bookmaker would multiply both sides to get a whole number, so for example 0.5:1 is the same as 1:2, or as the bookie might say "Two to one against". When the odds ratio is 2, the odds would be 2:1, and the bookie might say "Two to one on"

If x + y = total women in study attempting VBAC
and x = those who have a VBAC
y = those who attempt VBAC but end up with a repeat c/s,

Then the percentage of women who have a vbac is x/(x+y) x 100 The odds ratio of having a VBAC is x/y

So out of 5 women, if 4 have a VBAC and one ends up with a caesarean, then 4/5 x 100 = 80% have VBACs, and the odds ratio for VBAC is 4/1 = 4.

Return to Chances of a VBAC page

Home Birth Reference Page
Site Contents