Do women really own less than 2 percent of the world’s land? Do women constitute 70 percent of the world’s poor? Do women provide between 60 percent and 80 percent of the agricultural labor in Africa?

Do widely cited statistics like these mean they are backed up with solid research?

No, but they are repeated often enough that they have attained the status of official fact. They are referred to as “zombie statistics” because, though they have no basis in fact, they just won’t die. Numbers like these make good headlines and rally advocates for various causes. But if you’re trying to solve a problem, you have to know what the problem is, and zombies just muddy the waters.

Zombie statistics actually can be their own worst enemy. As Cheryl Doss, development economist at Oxford University, wrote in a 2014 blog, “Using unsubstantiated data for advocacy is counterproductive. Advocates lose credibility by making claims that are inaccurate and slow down progress towards achieving their goals because without credible data, they also can’t measure changes.”

In her blog, Doss assessed the credibility of the claim that women own less than 2 percent of the world’s land. Her research, she said, shows that the issue of landownership is far more complicated and diverse than can be reflected in one global statistic. For example, consider whether the statistic includes a woman’s sole ownership of land or a husband and wife’s joint ownership.

Women’s land rights are important, Doss says, but flawed data won’t resolve the issue. In fact, there are no precise global figures in which every country is represented with recent, sex-disaggregated data. She and other experts agree that women own less land than men do, but her research found that on average, across 10 African countries, 39 percent of women and 48 percent of men report owning land, individually and jointly. Looking at just individual ownership, the numbers drop to 12 percent of women and 31 percent of men.

Another widely cited statistical zombie is that African women supply 60 percent to 80 percent of agricultural labor on the continent. In a recent study that compiled surveys across six sub-Saharan African countries comprising 40 percent of the region’s population and an array of agro-ecological zones, the average female labor share in crop production was 40 percent, with wide disparities within countries. As agriculture is an important sector of Africa’s economy, it’s also important to understand how the sector operates in order to increase productivity.

A different zombie awoke when Carly Fiorina, a few months before she entered the GOP presidential campaign, said that “70 percent of the people living in abject poverty are women.” PunditFact, which along with PolitiFact is a nonpartisan fact-checking website run by editors and reporters from the Tampa Bay Times, an independent newspaper in Florida owned by the nonprofit Poynter Institute, traced the statistic back to a 1995 Human Development Report that used the figure a couple of times in its text but not in any of its data tables. PunditFact said staff at the United Nations Development Program, which produces the report, said they no longer use the figure, but it still pops up occasionally.

Another effort to attack zombies includes The Washington Post Fact Checker’s debunking of a claim that 300,000 children are used as child soldiers, which is based on an estimate that is about 20 years old and described at the time as a “best guess.”

Zombies seem to awaken in cycles, usually depending on needs at the time. But since “fake news” and “alternative facts” have grabbed public attention, we also seem to have more online fact-checkers, and those are good places to turn if you want to check data. In addition to the ones mentioned above are many others, including, IndiaSpend’s fact checker, and AfricaCheck.

Be skeptical of data that seem to size up a complex problem perfectly. “After all, research, policies, and investments can only be as good and effective as the data and evidence informing them,” say the authors of the recent study Agriculture in Africa—Telling Myths From Facts: A Synthesis. “Evidence-based policymaking requires sound facts as well as sound inference. With either one of them missing, researchers and policymakers alike risk flying blind.”