Always put a number into context

Always put a number into context

Always put numbers into context.

Alloporus has advised to never leave a number alone not least because 

Comparison is always critical when dealing with numbers. On its own, a number makes no sense, it’s naked, self-conscious and insecure. It needs some context for clothing and some friends to compare against.

Makes good sense. 

50 bananas are way too many for one family but nowhere near enough for the local greengrocer.

What then to make of two numbers quoted in a study of feral cats

2.4 billion birds

12.3 billion mammals

2,400,000,000 must be a large number.

Any number with that many zeros must be important.

So I went to the source of the number, the research article

 Loss, S. R., Will, T., & Marra, P. P. (2013). The impact of free-ranging domestic cats on wildlife of the United States. Nature communications, 4(1), 1-8.

In the abstract, we find that our large number is actually the median value in a range from 1.3 to 4 billion birds killed by cats. 

In other words, the real number could be 1.1 billion smaller or 1.6 billion larger than the one in the quote. There is a distribution of likely true values for the number of birds killed by cats.

The 2.4 billion number was calculated after gathering all the reliable evidence on predation rates by cats from the research literature and multiplying this by estimates of cat abundance across the US. 

More technically, “We estimated wildlife mortality in the contiguous United States by multiplying data-derived probability distributions of predation rates by distributions of estimated cat abundance” (Loss et al 2013). The probability distributions were generated by repeating this calculation 10,000 times using random selections (random draw) of the predation rates reported in the literature. 

Alright, now we are getting somewhere. 

The context is that the key parameters to generate 2.4 billion are predation rate and cat abundance and both these values come from a range reported in the research literature.

We also learn that predation is by predominantly feral cats, that is cats that exist in the wild and feed themselves by doing what cats do best — catch and eat small prey. 

It is the unowned (feral) cats that do the heavy lifting to generate the number and not the moggy taking a saunter out of the cat flap once in a while.

So far so good. 

Now we know where the number came from and can assume that the best available evidence was used to set the parameters in a simple multiplication — predation rate x number of cats.

The number is still naked

Knowing where 2.4 billion came from does not give us the full context.

What we need to know is how many birds are there? How many birds die each year from other causes than predation by cats? 

Then we might ask the really important questions.

  • Are the numbers of birds changing over time? 
  • Is any change in numbers over time (the trend) due to predation by cats?
  • Are there any other reasons why bird numbers might change?

You see 2.4 billion is likely a small proportion of the total number of birds alive in the continental US and perhaps even a small proportion of those that die each year. 

Small temperate-zone songbirds have a life expectancy of around 10 months. This means that many birds in the backyard do not last a year but persist through their progeny. If the bird that gets caught by a cat had already reproduced then it makes n material difference to the bird population.

Equally, there are other predators out there that eat songbirds, notably other birds. Raptors (eagles, owls and hawks) that eat adult birds and a host of bird and mammal species that raid nests for eggs and chicks.

Predation by cats is just another risk. 

Always seek context 

2.4 billion sounds like a very big number and it might be.

We don’t know if it is or if something should be done to change it unless the context is understood.

Any number quoted in isolation and especially those used to provoke an emotional response is naked and lacks significance without all the extra information around it.

Look for the context before taking any number seriously.


Hero image from photo by Anton Darius on Unsplash

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