# Presentation: 190565

## MEASURING CONTRACEPTIVE FAILURE

- James Trussell
- Office of Population Research
- Princeton University

## Issues

- Efficacy *versus* effectiveness
- Typical *versus* perfect use
- Pearl index *versus* life table
- Non-completion of trial
- Common errors in literature
- Results from literature
- Communicating the risk of failure

## Efficacy versus Effectiveness

- Efficacy: how well a method works under ideal circumstances
- Effectiveness: how well a method works in the real world
- Efficacy would be measured in a clinical trial whereas effectiveness would be measured in a survey or a chart review

## Sources of Data

- Surveys: NSFG 1973, 1976, 1982, 1988, 1995, 2002
- Nationally representative
- Retrospective
- Underreporting of abortion
- Overreporting of contraceptive failure leading to birth?
- Clinical trials
- Hawthorne effect and inference beyond trial setting
- Cycles of perfect use can be identified and pregnancy rates during perfect use can be estimated but adherence is self-reported

## Results from the 1995 NSFG:% becoming pregnant in the first year of use uncorrected and corrected for underreporting of abortion

| Method | Uncorrected | Corrected |
| --- | --- | --- |
| Pill | 7.3 | 8.5 |
| Condom | 9.7 | 14.9 |
| Spermicides | 16.6 | 28.2 |

- Fu. *Fam Plan Perspect* 1999;**31**:56-63

## Self-Reporting of Adherence

- Self reports on missed OCs compared with electronic recording on punched pills among 103 women for 3 cycles
- Agreement on only 45% of days
- Overreporting of no missed pills (53-59% *versus* 19-33%)
- Underreporting of missing 3+ pills (10-14% *versus* 30-51%)

- Potter. *Fam Plan Perspect* 1996;**28**:154-8

## Typical Use versus Perfect Use

- Contraceptive failure during typical use can be measured in a clinical trial or in a survey
- Contraceptive failure during perfect use has been measured only in clinical trials, since retrospective reporting of adherence in surveys is likely to be terrible

## What Is Typical Use?

- By definition, a woman is a user whenever she considers herself to be using a method
- Hence, typical use of a barrier method does not imply that it is actually used at every act of intercourse
- Typical use includes both inconsistent use and incorrect use

## What Is Perfect Use?

- By definition, perfect use of a method requires actual use according to the directions for that method
- Perfect use of  a barrier method requires that it be used correctly at every act of intercourse
- Perfect use does not imply no pregnancies

## Logical Error Ingrained in Literature

- Suppose in a contraceptive trial there are 100 years of exposure to risk of pregnancy
- 15 pregnancies occur during a cycle of imperfect use
- 5 pregnancies occur during a cycle of perfect use
- What is the method-related pregnancy rate (pregnancy rate during perfect use)?

## Method-Related Pregnancy Rate

- Traditional answer
- 5/100 = 5 per 100 woman-years of exposure
- Logical error
- Denominator cannot be all exposure since by definition a method-related pregnancy can occur only during perfect use
- If there are only 50 woman-years of perfect use, correct answer is 5/50 = 10 per 100 woman-years of exposure

## Flaw in Design of Clinical Trials

- Information on perfect (correct and consistent) use is usually obtained only for cycles when pregnancy occurred
- Hence, pregnancy rates during perfect use cannot be estimated in most trials

## Correct Analysis by Cycle

- Woman 1: █ █ █ █ █ █
- Woman 2: █ █ █ █ █
- Woman 3: █ █ █ █
- Woman 4: █ █ █
- Cycles: imperfect use   	perfect use	all
- Rates:  1/3 			1/15			2/18

**P**

**P**

- P = pregnancy

## Pearl Index versus Life Table

- Pearl index is a pregnancy rate: pregnancies per 100 woman-years *of exposure*
- Ranges from 0 to 1300, not 0 to 100
- Rubber yardstick: women most likely to become pregnant do so early, leaving behind a pool increasing consisting of the more compliant and less fecund
- Life table methods produce estimates of the percent of women becoming pregnant within specific durations (*e.g*. 6 or 12 or 24 months) since initiating use

## Pearl Index Is a Rubber Yardstick

- Two investigators using the same data obtained pregnancy rates of 7.5 and 4.4 during 100 woman-years of condom use
- One (who got 4.4) allowed each women to contribute up to 5 years of exposure whereas the other (who got 7.5) allowed each women to contribute only up to 1 year of exposure
- Which is correct?

## Non-Completion of Trial

- In the ideal trial, all women would either become pregnant or complete the trial without becoming pregnant
- But in fact a high fraction stop for other reasons (LFU, medical reasons, personal reasons)
- What is the consequence?

## Consequence of Non-Completion of Trial

- In life-table analysis, those who are censored are assumed to have the same failure rate as those who are observed, probably resulting in a downward bias
- In a Pearl index, more complex to assess: if those who are censored would have had a higher risk of pregnancy, observed Pearl index could be biased upward or downward

## Example: Effect of Non-Completion

- 100 women start trial, 10 become pregnant at 6 months
- 50 women followed after 6 months, 1 of whom becomes pregnant at 12 months
- 40 women LFU at 6 months, 2 of whom would have become pregnant at 12 months
- Observed Pearl index = (10+1)/(50+25)=14.7
- True Pearl index = (10+1+2)/(50+25+20)=13.7
- Observed life-table probability=11.8%
- True life-table probability=13%

## Factors That Influence Failure

- Inherent efficacy of the method
- Imperfect use, the extent of which will depend on motivation to avoid pregnancy
- Frequency of intercourse, which declines with both age and marital duration
- Individual level of fecundity, which also declines with age
- Competence (honesty?) of the investigator

## Common Errors

- Incorrect calculation of method failure
- Multiplying cycles by 1200 instead of 1300 to get pregnancies per 100 woman years of exposure
- Not including “learning” phase
- Discontinuing non-adherent women

## Common Problems

- High percent not completing trial
- Underreporting of abortion
- Use of Pearl index when comparing risk of failure among methods

## Problems in Comparing Methods

- Results come from different sources; results where available for typical use come from the NSFG, adjusted for underreporting of abortion
- Women choose which method to use and are not randomly assigned to methods. Women who choose to use spermicides are very different from those who choose to use IUDs

## Results from Literature

- Table 27-1 from the 2007 edition of *Contraceptive Technology*
- Estimates of the annual risk of contraceptive failure during perfect and typical use of no method, spermicides, withdrawal, fertility awareness based methods, sponge, diaphragm, male and female condom, pill, patch, ring, injectable, IUD, implant, tubal ligation, and vasectomy

## What Table 27-1 Shows

- Methods requiring adherence generally show a big difference between perfect-use and typical-use failure rates
- The most effective methods during typical use are those not requiring adherence
- The most effective methods are not those that protect against STIs

## Communicating Risk of Failure

- Two studies have examined
- How well do women understand contraceptive failure rates?
- How to communicate contraceptive effectiveness?
- Result is a chart that appears in the new WHO *Global Handbook for Family Planning Providers * and the new edition of *Contraceptive Technology*

- Steiner. *Obstet Gynecol* 1996;**88**:24S-30S
- Steiner. *Obstet Gynecol* 2003;**102**:709-17

## Comparing typical effectiveness of contraceptive methods

**Comparing typical effectiveness of contraceptive methods**

**More effective**

**Less effective**

*Less than 1 pregnancy per 100 women in one year*

*About 30 pregnancies per 100 women in one year*

_**Injections**_**: Get repeat injections on time **
_**LAM**_** (for 6 months**_**)**_**: Breastfeed often, day and night**
_**Pills**_**: Take a pill each day**
_**Patch, ring**_**: Keep in place, change on time**

_**Condoms, diaphragm, sponge**_**: Use correctly every time you have sex**
_**Fertility-awareness based methods**_**: Abstain or use condoms on fertile days. **** **Newest methods (Standard Days Method and TwoDay Method) may be the easiest to use.

**How to make your method most effective**

**After procedure, little or nothing to do or remember**
_**Vasectomy**_**: Use another method for first 3 months **

_**Withdrawal, spermicide**_**: Use correctly every time you have sex**

**Injectables**

**Pills**

**LAM**

**Male Condoms**

**Female Condoms**

**Diaphragm**

**Spermicides**

**IUD**

**Female ****Sterilization**** **

**Patch**

**Ring**

**Fertility-Awareness Based Methods**

**Withdrawal**

**Implant**

**Sponge**

- Source WHO 2006, adapted with permission