As the 2016 Paralympic Games take place in Rio de Janeiro from Sept. 7 through 18, U.S. News & World Report is looking at the challenges facing disabled people worldwide.

Nearly 4 million people move through the Shinjuku metro station in Tokyo each day, finding their way through a maze of passageways to dozens of tracks on multiple platforms. The station is one of the world’s busiest transportation hubs, and on Sept. 10, 1986, it became a center of a building global disability rights movement.

Hundreds of people, many assisted by wheelchairs and walkers, paraded through the bustling streets of Tokyo shouting, “Access now!” These members of the Japan chapter of Disabled Peoples’ International, led by Shoji Nakanishi, planned to travel from Shinjuku to the Ueno Park station, where many, if not all, knew they would find themselves stuck underground. The Ueno Park station, like nearly all of Japan’s public transportation at the time, was not accessible for most people with disability.

In staging this radical protest and others like it, Nakanishi, who lost the use of his arms and legs after a motorcycle accident, has contributed to a global effort to empower people with disability and foster inclusive societies. The perceived power of this movement in Japan was in showcasing the sheer number of people affected. But 30 years later, while most of Japan’s stations now meet accessibility needs, hard numbers related to disability — the statistics that could help drive similar change around the world — have yet to be pinned down.

Disability statistics on a global scale are almost nonexistent, experts say. Efforts to collect, standardize and analyze international data have been scattered, largely due to challenges defining disability and to ingrained discrimination that has made disabled people less of a priority in policymaking.

Data doesn’t make systematic change on its own, experts say, but it can be the impetus to draw attention to the problems and help identify gaps in policy.

“Data and statistics represent reality,” says Ricardo Pla Cordero, an inclusion adviser for Handicap International’s humanitarian efforts. “One example can move emotions, but response programs are moved by data.”

Many global policymakers presume efforts focused on improving the lives of people with disability yield minimal results, both for the individual and society, says Judith E. Heumann, special adviser for international disability rights at the U.S. Department of State.

High levels of shame and stigma attached to disability cause underreporting, by both individuals who resist acknowledging their limitations and by governments that don’t consider people with disability when conducting census and other surveys.

On a basic level, the logistics behind segregating disability data are simply more difficult than those for gender, race or other commonly tracked demographics, experts and advocates say. With such a wide variety of individual experiences potentially falling under the same umbrella, determining the percentage of people with a disability within a population presents complications. Prevalence estimates vary widely from country to country, ranging from 24 percent in New Zealand to 1.5 percent in Iran, according to the U.N. Economic and Social Commission for Asia and the Pacific in 2015.

Many experts agree that defining disability presents unique challenges to disability data collection. The definition of disability is an “evolving concept,” says Charlotte McClain-Nhlapo, disability adviser for the World Bank.

But people will always want something to compare, and drawing conclusions on an international level is difficult when these figures are calculated with different goals in mind, says Daniel Mont, co-president of the Center for Inclusive Policy and a member of the Washington Group on Disability Statistics. The “correct” figure countries may use to fund eligible disability pension funds has a much different methodology behind it, for example, than that used to understand the percentage of people who are unemployed because of their disability, he says.

The most commonly cited figure for global prevalence of disability is in the 2011 World Report on Disability by the World Health Organization and World Bank: About 15 percent of the world’s population — more than 1 billion people — lives with disability. But it’s an estimate based on country surveys from more than a decade ago, each of which used a slightly different definition of disability.

McClain-Nhlapo says that there haven’t been new reports because there simply hasn’t been transformative new data. Others cite funding as the roadblock.

Nonetheless, advocates agree that great strides have been made recently in the push for new international data tied to disability trends, especially in the last decade. But the path hasn’t always been clear.

In 2000, the United Nations adopted the Millennium Development Goals, “quantified targets for addressing extreme poverty” and “promoting gender equality, education and environmental sustainability” that were to be achieved by 2015.

In the eyes of disability advocates, the goals had a tragic flaw: People with disability were an afterthought, if they were thought of at all.

“This was the framework that guided the work of the United Nations in the last 15 years, but disability was not mentioned, so it was not considered,” says Maria Martinho, social affairs officer at the U.N.’s Division for Social Policy and Development.

In 2006, in what Martinho calls a “breakthrough moment,” representatives from Mexico proposed that the United Nations General Assembly build a committee to draft a treaty to protect the rights of people with disability, as they had for women and children.

“They put forth the economic argument that if we include people with disability, we would be able to achieve a greater good for society as a whole,” she says. One year later, the Convention on the Rights of Persons with Disabilities was opened for signatures of member states.

The convention is a legally binding document for those countries that sign it, but it is not too prescriptive. The real change is meant to happen at the national level, as member states implement legislation that conforms to the convention’s guidelines.

Data collection has its own section, Article 31, which states that required information “shall be disaggregated, as appropriate, and used to help … identify and address the barriers faced by persons with disabilities in exercising their rights” and made available to the public.

Progress reports — due from countries two years after signing and every four years after that — continue to trickle in. Martinho, who serves as secretariat for the convention, says she thinks the next 15 years will be a “much better situation” than the last 15 years. Some change is already evident.

In 2015, when the U.N. adopted its next set of guiding principles — the Sustainable Development Goals — they were more inclusive of disability. Of the 17 goals, 164 targets and hundreds of specific statistical indicators that are set to be achieved by 2030, disability is mentioned explicitly in seven targets and 11 indicators and as a suggested focus point wherever relevant.

The first assessment report on the new goals is due to the General Assembly in 2018. While it is early to compare individual countries on their progress, experts say the initial assessment should help define the key issues and guide the next 12 years of implementation.

The challenge, experts say, is that to successfully standardize international data collection, defining a common purpose is key.

To help foster this necessary unity, the United Nations Statistics Division organized the Washington Group on Disability Statistics, an international group of diplomats and policymakers charged with developing a universal set of disability measures to be recommended for use in censuses and other national surveys.

The short set of Washington Group questions asks census or survey participants if they experience difficulties in five categories: vision, hearing, mobility, communication or cognition. Avoiding the blanket term “disabled” and offering respondents a range of response options beyond simple ‘yes’ or ‘no,’ says Mont, reduces stigma and increases response rates, helping to build a strong baseline to drive policy.

Still, as more and more international organizations give people with disability their attention, tensions persist among disability advocates themselves. They grapple over how to balance the medical model, which calls for cures and caregiving, and the human rights model, which views disability as an ever-present layer of diversity that should be celebrated as part of the human experience.

In any case, every person is entitled to basic human rights, experts say, and deserves to be an included member of society.

In some places, this is already reality. Nakanishi, now 72, carries his Japanese disability identification card with pride and enjoys the discounts it affords him on bus passes, theater tickets, medical care and more.

“Asian culture is not a strict one,” he says. “It is always to make harmony with the community.” As long as there are strong global leaders, he adds, the adoption of similar practices could benefit people with disability around the world.

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Lack of International Data on Disability Issues Hampers Advocates originally appeared on usnews.com

Update 09/07/16: