I ran away from home when I was 15. When I did, I removed myself from the radar of official statistics and counting. The last thing I wanted was to be noticed, noted and counted: my aim was to be quiet, on my own, invisible. But I was lucky, when I calmed down and re-established contact with my parents, I was still in the system. When I needed medical attention—which I did—the National Health Service still had me on their files. When I wanted to re-join the schooling system—which I did—I was registered and able to access free secondary schooling and, later, a university education.

But when I started working, I met young people in my position who happen to have grown up in countries where the system doesn't work so well. I am thinking of young people at a drop-in centre for drug addicts in central Europe, or children living on the street in Nairobi. What chance do they have to get back into the system, to get support from the state? Even if, in the case of central Europe, they were registered at birth, they are so far out among the "uncounted" that it is a long, hard journey to get back into the system, to get medical, educational and other support. And who is looking out for them and protecting them from violence? Sometimes we don't like to be counted and categorized, but the dangers of being uncounted are far worse.

I could so easily have been one of those children, so I have a personal "stake" in pursuing the objective that no child should be left behind. It is precisely those who are left behind that require our attention; yet it is precisely those children who are likely to be uncounted, not to be in the averages and national statistics that we use so often. The focus in the SDGs to leave no-one behind, and the thematic area on the same topic at the UN World Data Forum 2018 are crucial. Making sure we count everyone is a combination of technical, political and financial issues. It's technical because some of the groups we most want to monitor are not captured by regular statistical systems, such as administrative data or surveys. It's political because some groups are not captured due to discrimination and bias in the systems. And it's financial because more detailed statistics require greater funding to capture them. The World Data Forum is a superb place to bring all these issues, and the opportunities that new data sources and collection methods might offer, together for progress.

So much work remains to be done. The recent UNICEF report Progress for Every Child in the SDG Era identifies that one third of all children live in countries where there are insufficient data on SDGs related to children, and a further 1/3 live in countries where progress is insufficient to meet global targets. Insurmountable as this may seem, we're not starting from scratch and we have many tools at our disposal. The MICS survey programme is designed to build capacity in national statistics systems to capture many disparities, as we develop administrative data systems that are inclusive and able to identify those left behind.

If children are to count on us, we must first count them!

Moving forward, there are three directions we propose:

1. We have to establish data as the spine of system strengthening. As service delivery systems are built, we must ensure that data collection and capacity are equally strengthened as an inseparable part of these systems.
2. Beyond "leave no child behind", we must leave no country behind. Global support to data monitoring and capacity resembles a messy patchwork. We need systematic and coordinated efforts to ensure all countries have minimum data coverage for children, irrespective of their resources and capabilities.
3. And we need shared norms, beginning with open data. Common approaches are needed to measure emerging threats facing children, capture missing child populations such as those in institutions or migrating, and share data to enable vulnerable children to be more effectively identified, while protecting their privacy.

I am going to the UN World Data Forum 2018 to help advocate for, find partners for, and build on those principles so all children can have global, regional, country and local advocates and champions.

Mark Hereward is the Associate Director for Data & Analytics at UNICEF.