Converting between real and~percent is trivial: $$ \begin{align} r &= p\over100; p &= 100r. \end{align} $$ ^[Percent difference] (also called ^[relative change]) describes the~change in value as a~percentage (for example: $10$~is $50%$ of~$20$, and is~$100%$ of~$5$). ^[Percent change] recognizes the direction of the change (that is---$10$ is~$20$ reduced by~$50%$, so $p=-50$). Since ``change'' sounds like a~verb eclarative contexts, we use the~term ^[percent difference] to mean ^[relative change]; it is defined as: $$\delta \over x = {{x_\beta - x_\alpha}\over{x_\alpha}} \times 100 = p.$$ Notice that this preserves the direction of the change. So, in the context of the previous example, we can say that $10$ \emph{reduces}~$20$ by~$50%$, giving~$p=-50%$.