Contraceptive effectiveness 


Efficacy and effectiveness
Trussell and colleagues [1] define efficacy as being the inherent protection of the method in perfectuse cycles only, borrowing the term from epidemiology. Effectiveness is to do with typical use for all cycles  and therefore includes issues like compliance and has more 'realworld' utility, if, of course, effectiveness can be measured.
So condoms or pills may have high efficacy, but because people use them improperly, or forget to take them, the effectiveness of the method falls, and more pregnancies occur than would be expected. Oral contraceptives have high efficacy  but there are suggestions that effectiveness is much lower than the 99.9% efficacy often quoted.
Variables
There are many variables affecting conception and pregnancy rates  the capacity to conceive, frequency and timing of intercourse and the degree of compliance with a contraceptive method. Not all of the underlying nuances are captured by these broad terms, but one thing is likely to be universally true, and that is that the effectiveness of a contraceptive method will be lower than its efficacy.
Impact of effectiveness
Bandolier turned the question of effectiveness around, to know what level of effectiveness would provide what degree of protection against pregnancy. This was done for a couple where the woman had 13 cycles a year with four fertile days per cycle (52 fertile days a year), where intercourse happened twice a week, and in which only 1 in 5 conceptions led to a pregnancy.
By applying different rates of contraceptive effectiveness it is then possible to calculate (Table) how long (on average) it might be until a pregnancy occurs, or what are the odds of a pregnancy in any one year. Thus using a contraceptive with 75% effectiveness the odds are even (1:1) that a woman will be pregnant in any one year. Odds of 10:1 against being pregnant don't happen until effectiveness rates reach 97%, and 100:1 against until effectiveness is 99.7%.
Table: Bandolier's calculations of the impact of contraceptive effectiveness on the chance of a woman becoming pregnant in any one year
How long does it take to become pregnant?  
Days a year able to conceive  Contraceptive effectiveness (%)  Days  Months  Years  Odds of pregnancy in any one year 
15  1  122  4  0.3  1:3 
15  50  243  8  0.7  1:2 
15  75  487  16  1.3  1:1 
15  90  1217  39  3.3  3:1 
15  95  2433  78  6.5  7:1 
15  96  3042  98  8.2  9:1 
15  97  4056  131  10.9  11:1 
15  98  6083  196  16.4  16:1 
15  99  12167  392  32.7  33:1 
15  99.5  24333  785  65.4  65:1 
15  99.7  40556  1308  109.0  110:1 
15  99.8  60833  1962  163.5  165:1 
15  99.9  121667  3925  327.1  335:1 
Assumption 1: 13 cycles/year  
Assumption 2: 4 fertile days/cycle  
Assumption 3: 4x13 = 52 fertile days/year = odds of 1 in 7  
Assumption 4: sex on average twice a week = odds of 1 in 3.5  
Assumption 5: 1 in 5 conceptions lead to pregnancy  
Chance of conception for any one day without contraception = 1/7 x 1/3.5 = 1 in 24  
Chance of conception in any one year without contraception = 365/24 = 15 chances total 