Taking your 99% effectiveness of the pill, the probability of the failure of that is 1%, or 0.01. The probability of failure of the pull-out method (done correctly) is 4%, or 0.04. Combining the two, means the probability of simultaneous failure is 0.01*0.04 = 0.0004, or 0.04%, or 1 out of 2500 times.
This is in the form of retinal activity equivalents to account for the various sources of vitamin A. The best sources of preformed vitamin A are in liver and fish oils. This includes beef, calf, chicken liver and cod liver oil.
Higher doses might be used to treat vitamin A deficiencies, but you should never take more unless advised to do so by your doctor. Vitamin A food sources. Good food sources of retinoid vitamin A include: Eggs; Cheese; Yoghurt; Fortified spreads; Plant sources of vitamin A (from beta-carotene) include carrots, spinach, and apricots. Vitamin A supplement information. Vitamin A, often in the form of beta-carotene, is standard in multivitamins (it is omitted from pregnancy multivitamins).