FP2020 seeks to enable an additional 120 million women to use modern contraception by 2020. In order to measure additional users of modern contraception, the total number of modern users in each year must be compared to a baseline number of users. For the case of FP2020, the baseline is set to 2012, the year the initiative began.

Having an accurate estimate of the number of women who were using modern contraception in 2012 is key to measuring the success of FP2020. A Rolling Baseline allows for more accurate estimates of contraceptive use in 2012.

As more data becomes available each year, we can make better estimates of current contraceptive use, as well as better estimates of past use. Therefore, rather than measuring additional users from the original estimation of modern users across the FP2020 countries in 2012, we recalculate the full trend (starting with 2012) each year using the new and best data available.

We call this concept a “rolling” baseline because we re-estimate the baseline figure for 2012 each year.

There are two reasons the baseline estimate may change. First, a new survey has been released. There is often a delay of a year or longer before the surveys used to calculate mCPR are released. New data not only affects current year estimates, but those for prior years as well. For example (see graph), if a 2019 survey is not released until 2020 or 2021, the data used for FP2020 calculations would not be available when the 2019 estimates were prepared. When this data becomes available, this new survey will not only affect the estimate of mCPR in 2019, but also the trend in the years leading up to the survey.

Second, there is a new census that updates population estimates, specifically the number of women of reproductive age and the percentage of them that are married. Like the inclusion of new mCPR data, population data will change both the current and past estimates by influencing the trend between the last two censuses. Using a rolling baseline allows estimates to incorporate new data, ensuring they are the best estimate given all data available.

Continuously incorporating new data improves our ability to monitor progress, so that by the year 2020, our estimates for all years (2012 to 2020) will represent the most comprehensive and accurate data available.

The advantage of using rolling estimates is seen by comparing the original estimate of the number of modern contraception users that was calculated for the London Summit on Family Planning in 2012 (258 million), to the updated estimate for 2012 that we used in the 2017 Progress Report (270.5 million).

Our calculation incorporates new surveys and updated population estimates for 2012 that were not available at the time of the London Summit. As a result of this new information since 2012, we now estimate the total number of users in 2012 to be more than 3 million greater than was previously thought. Had we used the old estimate for 2012 as our baseline figure, it would overestimate the number additional users in 2019 by nearly 3 million (see graph).

Our re-estimation of the number of modern users in 2012 changes less each year, as now many countries have at least one post-2012 survey included in their projections. The largest shift in our baseline estimate was in 2013/2014, when many new surveys became available that were not yet published when developing the 2012 London Summit estimates.