Effectiveness Clarification

[sdm294]

I have seen throughout the articles on contraception that the rates are given as the possibility over one full year of use ---not on a per instance basis.

I know that these figures are trustworthy due to the scientific method being used throughout the studies, but is there any data or clarification you could provide as to the other variables involved?

-amount/frequency of sex
-use in conjunction with other methods (The Buddy System article was great, btw)
-age
-etc.

One single woman on the pill may only have had sex 5 times in a given year using condoms for additional protection while another woman in a committed relationship on the pill has sex every other day (180 times+) with her partner using no other means of contraception. Clearly, the second woman would theoretically be at a higher possibility of pregnancy even given the same method of contraception, although the overall rate would not reflect this important distinction.


For those users who only have sex a few times per year, it could be very reassuring to know that a typical rate of 91% is more along the lines of 98% due to the trend of users with very frequent sex causing the rate to swing downward.

If a birth control method is failing, it isn't due to some unknown force or mystery dice roll. There must be a mechanism of action (i.e condom breakage, antiobiotic interference with hormonal BC, etc.) and I have not seen much reflected in rates in more detail other than the "typical rates" category.

I'm not familiar with the particular statistical methods that were used in coming up with these particular rates, but if any additional information, studies, or resources could be provided it would be wonderful, if only for interest's sake only! Thank you

[Redskies]

I can clarify most of this for you, and then if you still feel like there are little gaps in what you want to know, I'm sure someone who's super-on-the-ball with current studies and scientific knowledge can add on later when they're available

Contraceptive effectiveness studies are done by observing thousands of couples using a method over the course of one year, and recording how many do and do not become pregnant using that method. So, single-method effectiveness rates are known through scientific study, not statistics. Our two-method Buddy System figures are arrived at through statistical methods using the original study figures, as explained in the piece. For the studies, the sheer number of couples involved almost guarantees a good representation of the broad range of sexual behaviour and the many possible variables, including frequency of intercourse: so, we can be very confident that the figures from studies do give a very accurate result for the overall population. There simply aren't any figures that take into account frequency of intercourse, for a couple of reasons: first, it'd be nigh-on impossible to conduct a study including it in a scientific manner, which really defeats the point of a scientific study; second, there's not actually a lot of point. Once we get to an individual level, wondering what one specific person's chance of pregnancy is, there are so many individual variables that you just couldn't do it - at least not with any kind of reliability or accuracy.

I don't know if you're very mathsy/statisticsy and wanting things like confidence intervals from those studies, or if you're wanting to see the summary of full content of a study? If you are, I'm sure someone who has a relevant study more to hand than I do would be able to point you in the right place.

As I touched on above, it'd only be of limited use to a person to know how much a difference in frequency would affect their own chance of pregnancy, simply because there are so very, very many things that can have an impact on an individual level. it's also something that no-one would be able to measure or talk about in an accurate way, which would rather defeat the point of having figures from scientific studies in the first place . I certainly wouldn't consider it appropriate or useful to start from a number that was arrived at scientifically and then do a guesstimation on it to produce a completely unscientific number; that kind of mixing of science and not-science seems misleading and likely to lead to less clarity, not more.

I'm not understanding what you're asking about when you say "use in conjunction with other methods". Our "Buddy System" piece makes clear the mathematical figures for two methods used together; one method doesn't impact the effectiveness of another method (with the exceptions of "don't use these together/take other precautions" for interactions like 'inside condom + outside condom' or 'Ella + combined pill', so those situations shouldn't happen anyway, and "take care to do it properly" for 'condom + withdrawal'). Can you clarify?

Age does factor into fertility: younger people, people in the age range we serve, as a population have higher fertility than the general population. From our Birth Control Bingo!: "a sexually active young adult who does not use contraceptives has an 85-90% chance of becoming pregnant within just one year." That's higher than the figure for the general population, which is 70-80%. I'm not aware of any studies on contraceptive effectiveness done exclusively on a young population, but again, someone super-current can confirm that.

Different methods have different reasons for failure, many of which are listed on our individual method pages within Birth Control Bingo!. "Typical use" rates allow for user error; "perfect use" rates are for laboratory conditions. The failure rate in perfect use may as well be a mystery dice roll, actually: "perfect use" means that the user did absolutely everything exactly as it was supposed to be, so there's nothing they could have done about it. Sometimes, there might be a discernible reason, for example, if a healthcare provider fitted a device incorrectly (though I'd stress that's exceptionally rare!); other times, there really isn't, and it's simply about a body being a body and somehow - again, rarely - getting through the contraceptive measures, for no known reason. (With condoms, most breakages are due to some kind of user error in usage or storage; the failures that happen in perfect use are breaks that happen when the user did absolutely everything properly. So, no-one needs to be concerned that a condom may just randomly fail and they won't know about it!) Does that clear that part up for you?