Evidence on failure rates is hard to come by. A series of estimates are summarised in a cost effectiveness paper , and the results are given in the Table both as the annual percentage of failure, and as the number of women (out of 10,000) who would become pregnant in one year using particular methods. Information on the combined oral contraceptive pill from a summary of clinical trials  is also given.
Annual failure rates for contraceptive methods given both as a percentage failure in one year, and in the number of pregnancies per 10,000 women over one year
|Method||Failure rate (%)||Pregnancies per 10,000 women per year|
|Combined oral contraceptive||0.08||8|
|From  except combined oral contraceptive, calculated from .|
This information on failure rates is probably less than perfect (for instance there was no definition of oral contraceptive). We were also perplexed as to how to use the information about Persona, which is claimed to be 94% effective. Does this mean that if 10,000 women not using contraception would have 8,500 pregnancies, with Persona this would be 8,500 x 0.06 = 510?
A review of clinical trials with the combined oral contraceptive of 30µg ethinyloestradiol and 150µg desogestrel  does not say it is systematic, but implies that it contains all the trials. Trials vary from small (200 women) to huge (>10,000), with wide rates of irregular bleeding and adverse effects. It is possible to perform an overall average, and this shows the following in cycle:
- breakthrough bleeding - 1.1%
- spotting - 3.3%
- amenorrhoea - 2.9%
- nausea - 1.5%
- headache - 2.9%
- breast tenderness - 3.6%
- nervousness - 3.9%
- J Trussell, JA Leveque, JD Koenig et al. The economic value of contraception: a comparison of 15 methods. American Journal of Public Health 1995 85:494-503.
- K Fotherby. Twelve years of clinical experience with an oral contraceptive containing 30µg ethinyloestradiol and 150µg desogestrel. Contraception 1995 51:3-12.