Do I look like Wikipedia?

Ipsos, the polling firm, has again been asking people questions to which they can’t reasonably be expected to know the answer, and finding they in fact don’t.  For example, this graph shows what happens when you ask people what proportion of their country are immigrants, ie, born in another country.  Everywhere except Singapore they overestimate the proportion, often by a lot.  New Zealand comes off fairly well here, with only slight underestimation. South Africa and Canada do quite badly. Indonesia, notably, has almost no immigrants but thinks it has 20%.

Some of this is almost certainly prejudice, but to be fair the only way you could know these numbers reliably would be if someone did a reliable national count and told you.  Just walking around Auckland you can’t tell accurately who is an immigrant in Auckland, and you certainly can’t walk around Auckland and tell how many immigrants there are in Ashburton.  Specifically, while you might know from your own knowledge how many immigrants there were in your part of the country, it would be very unusual for you to know this for the country as a whole.  You might expect, then, that the best-case response to surveys such as these would be an average proportion of immigrants over areas in the country, weighted by the population of those areas. If the proportion of immigrants is correlated with population density, that will be higher than the true nationwide proportion.

That is to say, if people in Auckland accurately estimate the proportion of immigrants in Auckland, and people in Wellington accurately estimate the proportion in Wellington, and people in Huntly accurately estimate the proportion in Huntly, and people in Central Otago accurately estimate the proportion in Central Otago, you don’t get an accurate nationwide estimate if areas with more people have a higher proportion of immigrants. Which, in New Zealand, they do.  If we work with regions and Census data, the correlation between population and proportion born overseas is about 50%.  That’s enough for about a 5 percentage point bias: we would expect to overestimate the proportion of immigrants by about 5 percentage points if everyone based their survey response on the true proportion in their part of the country.

Fortunately, if the proportion of immigrants in your neighbourhood or in the country as a whole matters to you, you don’t need to guess. Official statistics are useful! Someone has done an accurate national count, and while they probably didn’t tell you, they did put the number somewhere on the web for you to look up.