Douglas Adams' classic The Hitchhiker's Guide to the Galaxy offers salient advice about an awful lot of things. Don't listen to Vogon poetry. Don't forget where your towel is. And don't have too strong a sense of perspective because fully recognising our place as an insignificant dot on an insignificant dot against the vastness of all creation is mentally debilitating. 

What the Hitchhiker's Guide failed to note are the dangers of having too little sense of perspective. 

A fortnight ago, Keep New Zealand Beautiful released its National Litter Audit. Volunteers had scoured scores of sites across the country where people are likely to litter: car parks, roadsides and the like. They tallied the litter found at each site, compiling the litter that had accumulated over the years. And they cleared the sites so the flow of litter rather than the accumulated stock could be measured at the next audit.

So far so good.

But the organisation, to generate a big headline-grabbing figure, averaged the amount of litter found at sites likely to have litter and extrapolated it to the whole country as though all of New Zealand were comparable to the sites they surveyed. 

That does not work. Nearly 80 per cent of the country is effectively uninhabited, with a population density of less than a person per square kilometre. The middle of Fjordland is unlikely to have litter comparable to a roadside rest stop. 

That extrapolation generated big big numbers – 10 billion littered cigarette butts around the country, almost 395 million litres of littered disposable nappies, and the like.

Anyone with a sense of perspective would have known those numbers were fishy. 

The government collects just under $2 billion in tobacco excise per year, and excise on a cigarette is just under a dollar per stick, so it's likely around 2 billion cigarettes are smoked in the country per year. Is it likely that a country that smokes about 2 billion cigarettes per year has 10 billion littered cigarette butts strewn about? 

Similarly, is it likely that a country where about 60,000 kids are born annually can generate that volume of littered nappies? Suppose a littered nappy is about a litre, and that a baby uses eight nappies per day in its first year. That gives us just over 175 million nappies used by the country's newborns every year. How then is the volume of littered nappies more than double that figure? 

Nobody reporting on these figures seems to have a sense of perspective. Fortunately, Keep New Zealand Beautiful has, after no small amount of prodding from me, pulled the dodgy stats from its website and from its report – but has issued no formal correction or retraction. The numbers will live forever in uncorrected newspaper accounts and be cited in uncountable school essays and newspaper articles to come. 

All due to a lack of perspective. 

Simple back-of-the-envelope calculations based on numbers that, if not common knowledge, are easily found, would have set straight these claims. 

Here are a few more numbers worth memorising. They help in providing a sense of perspective. You can use them when benchmarking claims to see whether they make a lick of sense.

New Zealand has about 5 million people. When you see a big number for the country as a whole, dividing it by the population can help check whether the number seems plausible.

New Zealand's Gross Domestic Product (GDP), by the expenditure measure, was just under $300 billion for the year to March 2019. A 5 million population gives us a per capita GDP of about $60,000. 

The government's budget for 2019/2020 was just under $111b – the government spends about $22,000 per person. Just over $24 billion is spent on benefits through the Ministry of Social Development, with $15.5b of that going to NZ Super. The total health budget is about $18b. Police get just under $2b. Pharmac is about $1b. 

So when you hear claims that alcohol costs $7.8b per year, it becomes immediately obvious the figure isn't a measure of costs to the public health system or police – unless you're happy to believe that alcohol can be blamed for 40 per cent of total government spending in those areas. Rather, the number includes a lot of private costs like spending on alcohol – and as I've previously shown, counts some of those things twice. 

New Zealand is a big place: just under 27 million hectares. As of 2012, urban land comprised about 228,000 hectares: only about 0.85 per cent of the country is urban land. So we are in no danger of urban sprawl, or landfills, taking over the whole country.

A moderate, healthy sense of perspective is far from dangerous. 

It is vital? Yes, especially when journalists seem to err on the side of having none whatsoever.