There are also compromises in the way that the questions are asked. In an earlier More or Less programme, you might have heard an item on the UK Government’s International Passenger Survey (IPS). The IPS uses interviewers to collect data from travellers entering or leaving the UK, on things like travel plans, nationality, and amounts spent on accommodation. These data provide information on migration to and from the UK, economic information for the Government, useful information for the travel industry, and much more besides. The programme followed a team of IPS interviewers working on a cross-Channel ferry. They choose passengers on a particular random basis as they boarded the ship. But the interviews do not take place when these passengers are chosen; instead the interviewers note details of their clothing and so on, and then try to find them and interview them later, during the voyage. It isn’t always straightforward to find them again.

In a way this all sounds slightly chaotic and messy, and indeed the IPS interviewers don’t always find their chosen interviewees. But the question to ask yourself is not, “Is this a perfect way to collect the data?”, but “Is this the best way to collect the data, given all the constraints and restrictions involved?”

The interviewers could avoid having to go back and find people again, if they interviewed them while they were coming onto the ship - at the time they are chosen for the sample.

But that would disrupt the flow of passengers onto the ship, and possibly hold up the journey for everyone. It would annoy people who are just coming on board and are probably keen to find a place to sit or a cup of tea, and annoyed people are likely to turn down a request for an interview.

The IPS team could just give out survey questionnaires for the passengers to fill in, and hope that enough of them would be returned later.

But again, many people might not bother. Also there are particular issues of language on a survey involving international travel which could cause problems for a paper questionnaire like this.

Or the sample of people could be chosen during the voyage, rather than when people are coming aboard.

The trouble with this is that some people are going to be a lot easier than others for the interviewers to come upon during the voyage, because they are sitting in easily accessible parts of the ship. The interviewers would be more likely to select these easy-to-find people for interview, and such a sample would not be representative of the whole group of passengers. When passengers are coming on board, they all have to enter through a limited set of doors, and everyone can be treated on an equal basis.

So, while the IPS method isn’t ideal, it may well be a very good practicable method of gathering these data. IPS interviewers on sea crossings do obtain useable information from over 85% of the travellers that they sample, which is a much better response rate than most surveys achieve.

So, in a survey, you have to keep a lot of balls in the air. You have to ask enough people to take part, and you have to make sure that too many people don't turn you down. You have to make sure the people you ask are representative enough. You have to ask questions that don't bias the answers. And there's a lot more to take into account that I haven't had space to mention. The general idea might be like tasting a stew, but the whole thing is rather more complicated. With all this going on, no survey is going to give you perfect answers. The skill in conducting a good survey lies in making sure that the results are as good as they can be.

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