No, pre-drinking isn't more common among people aged over 30

The New Zealand Herald reported recently on some new (and interesting) cross-country research:

New research has found that New Zealanders increase their pre-drinking after the age of 30, instead of slowing down.

A University of Queensland study explored the pre-drinking habits of people in 27 countries and found in six countries, including New Zealand, pre-drinking appears to increase after the age of 30.

Pre-drinking, also known as pre-loading, pre-partying or pre-gaming, is most commonly defined as the consumption of alcohol in domestic settings prior to attending licensed premises.

The original research paper (ungated), by Jason Ferris (University of Queensland) and co-authors, was published recently in the journal Alcohol & Alcoholism. Having read the paper, it contains some interesting comparisons between countries in terms of reported pre-drinking. However, there is also plenty of reason for skepticism regarding the claim that New Zealanders increase their pre-drinking after age 30.

First, the data were from the Global Drug Survey, which Ferris et al. describe as:

...a non-probability sample of people who self-select to complete the online survey investigating past use of alcohol and other drugs...

In other words, it's basically an online poll that is not representative of the population. On top of that, they restrict the sample to exclude non-drinkers. So, their results don't show the proportion of people who are pre-drinkers, but instead show the proportion of drinkers who are pre-drinkers. If the proportion of people who drink is lower at higher ages, then their measure of pre-drinking prevalence will be biased upwards at higher ages because of their sample restrictions.

On a similar note, think about who is going to answer the Global Drug Survey. It seems to me that the sample itself may be biased towards those who drink more, drink more often, and pre-drink more. In that case, there is even more bias in the results.

Second (and pedantically), this is cross-sectional data, so it doesn't say anything about what people do as they get older. The data allow us to compare between different age groups, but are silent as to whether those who are currently aged under 30 will pre-drink more as they get older. So claiming that "New Zealanders increase their pre-drinking after the age of 30" is clearly misunderstanding how the results work (that is the journalist's issue though, not the researchers).

Third, even saying that people aged over 30 are pre-drinking more than younger people is pretty misleading. Here's the part of Figure 4 from the paper that relates to New Zealand:

The blue line tracks the proportion of men who reported being pre-drinkers, by age, while the pink line tracks the same proportion for women. You can probably see the uptick in pre-drinking after age 30 for both men and women, but it doesn't get back up to the peak in the early 20s. However, the error in these estimates gets larger in the higher age groups, and the bars are all overlapping, so clearly you'd be over-interpreting the data to claim there is a strong increase in pre-drinking at higher ages.

My own research from 2014 (which I blogged about here) actually shows that, within a representative sample of people in the night-time economy, pre-drinkers are younger (25 on average, compared with 31 on average for non-pre-drinkers), and that pre-drinking was strongly negative correlated with age. Even when I run a cubic of age (which is essentially what Ferris et al. did), I still get a downward slope of pre-drinking prevalence across all ages up to at least 40 (and then it's pretty flat). Here's the raw cubic regression line (for both genders combined):

Notice two things from this graph: (1) there is a massive drop-off in pre-drinking from young ages to older ages (that's because the sample here is representative, and not limited to drinkers only); (2) there is basically no up-tick in pre-drinking at older ages (unless you squint really hard after age 40, and even then the line is basically flat).

Research that Matt Roskruge and I did earlier this year (also based on a representative sample of people in the night-time economy) is also consistent with those 2014 results (and I'll blog on that research a bit later in the year).

Pre-drinking is a problem, because it is a major contributor to intoxication in the night-time economy (see my earlier blog post for more on that point). However, while many people aged over 30 do engage in pre-drinking, it is much more prevalent among younger people.

Read more:

• The temporal gradient of intoxication in the night-time economy