ON AUGUST 25th a group of militant Rohingya Muslims attacked police bases in northern Myanmar. The army retaliated with untrammelled fury, burning villages, killing civilians and raping women. More than 420,000 terrified Rohingyas have crossed the border into Bangladesh. The UN’s International Organisation for Migration (IOM) has proclaimed the exodus “unprecedented in terms of volume and speed”, and Zeid Ra'ad al-Hussein, the UN’s human-rights chief, called it a “textbook example of ethnic cleansing”.

Myanmar’s leaders deny they are conducting a campaign of repression against the Rohingyas. Aung San Suu Kyi, the de facto leader of the government and a winner of the Nobel peace prize, has repeatedly failed to condemn the attacks. Speaking on September 19th, she again avoided mentioning the Rohingyas by name, and flatly claimed that no violence or village clearances had occurred since September 5th. Amnesty International, a human-rights group, branded the speech “a mix of untruths and victim-blaming”.

Despite widespread international criticism, Ms Suu Kyi’s stance is widely shared in Buddhist-majority Myanmar. In 1982 the military government excluded the Rohingyas from a list of more than 130 officially recognised ethnic groups in the country, dismissing them as illegal immigrants from Bangladesh. That rendered them, in effect, stateless, and their mistreatment intensified. In 1991-92, around 600,000 Rohingyas fled across the border to escape violent persecution by the army.

The current exodus is unfolding much more swiftly. On average, 120,000 people have crossed the border per week, although the rate has recently started to slow. Aid agencies say they are overwhelmed and cannot provide enough food, water or shelter. Other refugee crises have involved a larger total number of refugees, but have stretched out over longer periods, sometimes lasting years, so the flow has been less intense than the exodus from Myanmar.

Data on weekly refugee flows are often unavailable. The Economist has attempted to estimate them by using UNHCR figures for selected crises and averaging the yearly flows over 52 weeks (when yearly data are available, eg, for Iraq and Syria), or by averaging the overall number of refugees over the relevant time period (eg, for Liberia and Afghanistan).

This suggests that the current refugee flow from Myanmar is swifter even than the exodus from Rwanda in 1994. Some 2.3m people fled the country, more than a third of the population. (Confusingly, the refugees were mostly not Tutsis (the targets of the genocide, who were largely unable to escape) but Hutus (the perpetrators). A Tutsi rebel army overthrew the genocidal Hutu government, and its leaders fled, taking a huge portion of their own tribe with them.) We have assumed that most of the Rwandan exodus occurred between April and August 1994. If so, an average of 111,000 Rwandans left the country every week.

The refugee crisis in Syria is the worst of the past decade. Some 5.5m people have left the country. But averaging yearly flows, about 33,000 people left the country every week in 2013, the worst year.

Until the most recent violence began in Myanmar, some 1m Rohingyas lived in Rakhine state. Nearly half have gone to Bangladesh, which already hosted around 400,000 Rohingyas from previous outflows. A further 700,000 live in other countries in Asia and the Middle East. Ms Suu Kyi has said her government is prepared to begin a verification process “at any time” that would allow some Rohingyas in Bangladesh to return home. How many will be allowed to come back remains to be seen.