About 1 or 2 in every 100 women who use ECPs will become pregnant despite taking ECPs within the recommended amount of time.
Statistically, about 5% - that's 5 out of 100 women - having totally unprotected sex - will conceive in a single act of intercourse.
Other studies have shown that the effective rate of EC is less (that site is using AMA stats):
EC is up to 89 percent effective in preventing pregnancy, depending on how quickly a woman begins the pills, the type of pills taken, and when the sex occurred during the woman’s menstrual cycle.
The state of Illinois has, also, some stats that would seem to back up the AMA stats:
Combined EC pills are about 75 percent effective. Progestin-only EC pills are 89 percent effective if used within 72 hours and 95 percent effective if used within 24 hours.
Why is this such a big deal? The common perception is that EC pills are a magic treatment that WILL prevent the pregnancy from starting. There is no easy way to undo the effects of a violent sex assault. A certain percentage of women will, in fact, become pregnant - about 5%, same as any other single act of unprotected sex. The way I've seen the statistics on effectiveness thrown around makes me quite suspicious of them - too many people will confuse the numbers, say that if 95% of rape victims don't get pregnant, then the treatment was 95% effective.
In fact, in such a case, the treatment had NO effect - about 5% would have become pregnant in any case.
Another concern is - what about the 95% who WOULDN'T have become pregnant? They are being given some very powerful hormones that may have long-term side effects. And, they are being pressured to take that treatment at a VERY vulnerable point in their lives, while they are quite traumatized from the attack.
That isn't exactly what I'd call informed consent.