Methodology of the International Passenger Survey
Interview teams are required to identify every 'nth'
person ('n' varies by port and route, taking account of traffic
flows) in the flow of passengers past a specified point. Information
is collected from any migrants identified through these routine
samples. However, for selected ports and routes, additional passengers
are selected for a short interview.
The questions asked are designed to establish whether
the people selected are migrants. If this is the case, more detailed
questions are then asked. Prior to 2007, these additional interviews
were carried out only for arrivals (to identify immigrants only).
The ONS has reported that since January, they have been extended
to departures (to boost the number of emigrants interviewed).
At present the only reliable source of information on emigration
is the IPS.
Sampling for the International Passenger Survey is
carried out at all airports with more than 1 million international
passengers travelling through them. This currently includes 5
London airports, Manchester and 10 regional airports. The IPS
uses data provided by the Civil Aviation Authority (CAA) to extrapolate
the sample figures to total flows, to ensure that all people are
accounted for. In addition the Channel Tunnel and 10 sea ports
are covered and again the survey data are weighted to total flows
supplied by Euro tunnel, Eurostar and Department for Transport.