Estimates of the number of overseas students who leave the UK after studying are painting a ¡°potentially misleading¡± picture of net migration and should be treated with caution, the country¡¯s statistics watchdog has warned.

, the Office for Statistics Regulation (OSR) warns that the estimates ¨C based on a survey of people entering and leaving the country ¨C?need to carry a warning due to the potential for error.

The data, published each quarter by the Office for National Statistics (ONS), have consistently suggested that far fewer overseas students are leaving the UK than arriving, leading to speculation that a major factor was students overstaying their visas.

In her previous role as home secretary, Prime Minister Theresa May?suggested that ¡°too many¡± students were overstaying in a speech that sought to justify the government¡¯s policy of making it tougher for students to gain visas.

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But in a , the director-general of the OSR, Ed Humpherson, says that the ¡°former-student emigration estimate does not bear the weight that is put on it in public debate¡±.

This is because it is based on responses to the International Passenger Survey (IPS), which, while helpful for making estimates of net migration overall, may not be reliable enough to make a judgement on student flows.

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¡°We therefore have asked [the] ONS to make clearer [in its quarterly statistics] that this estimate should be treated with caution,¡± Mr Humpherson¡¯s letter continues.

In its report, the watchdog says that the estimate ¡°creates doubts about the patterns of student migration and generates a narrative that is potentially misleading for a topic of major public interest and policy sensitivity¡±.

It adds that ¡°former-student emigration could reasonably be expected to be lower than student immigration, due to the range of outcomes of former students, but it is unclear whether it occurs on the scale seen in the ONS estimates¡±.

The body has told the ONS that it should make clear that previous and future estimates are ¡°experimental¡± data and has asked it to publish more contextual information about the statistics.

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Alistair Jarvis, acting chief executive of Universities UK, said that he was ¡°pleased¡± that the OSR had recognised the potential problems with the data and that the IPS ¡°was never intended to be an accurate measure of migration flows¡±.?

¡°We know from other data that only a very small percentage of international students failed to comply with their visa requirements,¡± he said.

James Pitman, a spokesman for Destination for Education, a group backed by private providers of university preparation courses for overseas students, also welcomed the report and said that immigration policy was being based on ¡°flawed data which does not accurately represent the nature of international students¡±.

The ONS is due to publish its next round of net migration statistics on 24 August.

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The OSR report was published on the same day that the government announced that it was asking the Migration Advisory Committee to produce a report on the costs and benefits of European Union migrants, including students and academics, that will inform the UK¡¯s post-Brexit immigration policy.

simon.baker@timeshighereducation.com