## The best proof-readers are either time...or maths teachers

How many times do you have to look at something before you spot it contains an error?

When I submitted the manuscript for 'Maths on the back of an envelope', it included a calculation that in effect says this:

30% of 40 = 12%.

It should of course say 12, not 12%.

How did I make this mistake in the first place?  Who knows, we all make mistakes, but one possibility is that I'd just been working on a different calculation involving units (maybe 4 x £12 = £48), and in a moment of absent-mindedness I treated '%' as being a unit like '£'.

But of course the idea with proof-reading is that errors like this get picked up.  To be fair, this wasn't the only error in the first draft - there were probably 200 points that we later spotted between us that needed correction or clarification and which we put right.  But somehow, the 12% made it through.

I was then sent back the proofs, still containing the error, and missed it.  And that's partly because when it's something you have written, there's a tendency to (a) believe what you have written is right, and (b) in any case, you often read what you expect to see, not what you actually see.  That's how it is for me, anyway.

One of the best techniques for proof-reading is to leave something for a week and then go back to it.  Often it works, but as this example shows, the method isn't foolproof.  Several other readers of the proofs missed it too. (There's a formula that predicts this, known as the Lincoln Index which I discussed in an earlier blog http://www.robeastaway.com/blog/the-lincoln-index ).

And so - on page 48 of the first edition of the book you will find the erroneous claim that 30% of 40 = 12% and that 9% of 80 = 7.2%.  Needless to say, several maths teachers reading the book have spotted it.  (And yes,  I found other typos when reading the book aloud recording the Audiobook - a '115' instead of '215', for example).

Typos/errors are extremely annoying, and my apologies to anyone who finds one and is caused a moment of confusion or irritation.  If there's a silver lining, these typos were cleared up in the second print run and on Kindle, so most readers won't have the, ahem, 'distinction' of having what is provably the first edition of the book.  And somewhat ironically, spotting errors in calculations is exactly what this book is looking to encourage.

But what I really think is:  Doh!