One thing to be aware of is that "perfect use" is theoretical. It's a figure determined by contraceptive researchers, based on various bits of evidence that determine the total likelihood, in a year, of method failure. (I.E. the method itself failing to act properly.) Oral contraceptives have generally had a listed perfect use efficacy rate of 99.8% or so, meaning about 2 women in 1,000 will get pregnant in a year of using just HBC.
"Typical use" -- which is usually pegged at about 92% efficacy in a year for HBC use, or about 8 in 100 women becoming pregnant in a year -- is the data taken from real populations, meaning that that's the number of pregnancies they see in a year in actual populations they have surveyed.
However, "typical use" stats also have their problems. For one, the measurements can be very poor at measuring multiple methods -- i.e. in many surveys, if two methods are used, they only count the most effective method.
On the other hand, with hormonal birth control pills, most pregnancies in the "typical use" surveys occured when pill users stopped taking pills entirely, but did not begin a new method at all. Their pregnancies are still counted against the pill because of the way the surveys work -- even though they weren't using the pill at the time.
One thing I have wondered -- and for the record I have NOT looked into this at all, but we had a discussion in the past -- is the LoEstrin stat actually represents the "typical use" stat minus the pregnancies occured when pill users quit but didn't switch. I ran the numbers once, and it worked out to bump that 92% typical use efficacy up to the 96% to 98% efficacy LoEstrin now names.
Anyway, it would make sense to me, but I can't comment exactly. But it's worth noting for your own understanding that "perfect use" has always been theoretical, and while in some ways "typical use" is more "realistic," it's also itself a highly problematic statistic.
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Date: 2011-08-04 03:04 am (UTC)One thing to be aware of is that "perfect use" is theoretical. It's a figure determined by contraceptive researchers, based on various bits of evidence that determine the total likelihood, in a year, of method failure. (I.E. the method itself failing to act properly.) Oral contraceptives have generally had a listed perfect use efficacy rate of 99.8% or so, meaning about 2 women in 1,000 will get pregnant in a year of using just HBC.
"Typical use" -- which is usually pegged at about 92% efficacy in a year for HBC use, or about 8 in 100 women becoming pregnant in a year -- is the data taken from real populations, meaning that that's the number of pregnancies they see in a year in actual populations they have surveyed.
However, "typical use" stats also have their problems. For one, the measurements can be very poor at measuring multiple methods -- i.e. in many surveys, if two methods are used, they only count the most effective method.
On the other hand, with hormonal birth control pills, most pregnancies in the "typical use" surveys occured when pill users stopped taking pills entirely, but did not begin a new method at all. Their pregnancies are still counted against the pill because of the way the surveys work -- even though they weren't using the pill at the time.
One thing I have wondered -- and for the record I have NOT looked into this at all, but we had a discussion in the past -- is the LoEstrin stat actually represents the "typical use" stat minus the pregnancies occured when pill users quit but didn't switch. I ran the numbers once, and it worked out to bump that 92% typical use efficacy up to the 96% to 98% efficacy LoEstrin now names.
Anyway, it would make sense to me, but I can't comment exactly. But it's worth noting for your own understanding that "perfect use" has always been theoretical, and while in some ways "typical use" is more "realistic," it's also itself a highly problematic statistic.