There is a strong negative correlation with the number of times a dessert was offered at a special price and the number of desserts sold. Either the lower the special price the more of that dessert is sold or the higher the price, the fewer number of those deserts are sold.
The are negative because in the first - the lower $ - the higher # of dessert
and the higher $, the lower. It is negative because lower and higher are opposites.
If it were a positive correlation the more they charged, the more are sold and the less they charge the less are sold (That of course, makes no sense).
