Sanity checking data

Published: Wednesday, 20 May 2020

WHEN I was an engineering apprentice in the last century, we had no calculators(!), and we used slide rules for calculations instead, writes Ralph Freeman.

Now using a slide rule had two problems.  There was no decimal point and it was possible to read the answer off 'the wrong end' or the wrong scale.  Therefore you were always taught to do a 'sanity check' on any result.

Declined dramatically

This practice of sanity checking data has stuck with me throughout my life.  Unfortunately, the use of this process, and the implementation of common sense appears to have declined dramatically over the last couple of decades which is not good. What has this got to do with the waterways?  Well here is an example of a sanity check.

Polluting the canals

I read an article a while back claiming the average canal boat deposits two litres of diesel/oil in the Cut per year.  Due to the lack of boating caused by Lockdown, I have found time to sanity check this figure which seemed hard to believe.  Read on:

Suppose Barton Marina (where I'm locked down) has 180 boats moored there.  (In fact it has many more, up to 300 max).
Taking 180 boats x 2 litres / year = 360 litres/year so that's approximately 1 litre/day of pollutants in the marina.
Looking on Google it says 1 quart of oil covers around 1 acre on water.

1 Quart = 1.13 litres and 1 Acre = 4046.8 square metres. This converts to 1 litre covers approx 3581 square metres

So that implies there would be an oil slick approximately 60 metres x 60 metres permanently present at the marina if the 2 litres/year boat discharge rate were true. That figure is a conservative one too, as there are more than 180 boats in the marina over the winter.


I am suggesting the pollution level of 2 litres/boat/year is well off the mark, as I have never moored (over winter) in any marina with that level of pollution.  I hope you will follow my reasoning above and in the future do your own 'sanity checks' on data presented by the media of any kind.  The results are often, shall we say, illuminating!