Percentages can be confusing

When I started training as an accountant, one of the first clients I visited was a bookseller.

I remember being unable to understand how the clerk had computed his percentages when sales were falling. These included such impossible results as minus 134% and minus 179%.

I explained how to compute the percentage movement in sales from one period to another. 

The look of joy on the clerk's face was a delight as he came to appreciate a key fact: 

• Whilst upward trends had no limit (eg: increases of four time the previous week's sales would show as plus 400%), downward trends could never be worse than minus 100% (if no sales were made). He was thrilled.

I went back a few weeks later to check all was well. Unfortunately a stack of books (previously counted as 'sold') had been returned, thus creating 'minus sales' ...............!