Later in the text, we explain in more detail what per mille means, what is a basis point and how to convert per milles and basis points to percents. Change in percentage points (pp): The difference between two percentage values. If one value is 10 % and the other is 30 %, the change is 20 percentage points (20 pp).

So what is percentage good for? As we wrote earlier, a percentage is a way to express a ratio. Say you are taking a graded exam. If we told you that you got 123 points, it really would not tell you anything. 123 out of what? Now, if we told you that you got 82%, this figure is more understandable information. Even if we told you, you got 123 out of 150; it's harder to feel how well you did. A week earlier, there was another exam, and you scored 195 of 250, or 78%. While it's hard to compare 128 of 150 to 195 of 250, it's easy to tell that an 82% score is better than 78%. Isn't the percent sign helpful? After all, it's the percentage that counts!We still don't know what the exact result is, so we take the exponent of both sides of the equation above with some change on the right side.

lg ( 5.89 × 4.73 ) = lg ( 5.89 ) + lg ( 4.73 ) ≅ 0.770115 + 0.6748611 \text{lg}(5.89 \times 4.73) =\text{lg}(5.89) + \text{lg}(4.73) ≅ 0.770115 + 0.6748611 lg ( 5.89 × 4.73 ) = lg ( 5.89 ) + lg ( 4.73 ) ≅ 0.770115 + 0.6748611 The other popular form of logarithm is the common logarithm with the base of 10, log₁₀x, which is conventionally denoted as lg(x). It is also known as the decimal logarithm, the decadic logarithm, the standard logarithm, or the Briggsian logarithm, named after Henry Briggs, an English mathematician who developed its use. The new computational procedure was instrumental in the field of astronomy. Napier's scientific activities coincided with the era of new developments in astrophysics. As a result, many astronomers were struggling with endless calculations to detect the position of the planets using Copernicus's theory of the solar system. Johannes Kepler, at the time working on his famous laws of planetary motions, was among them. Senator Homer Simpson was polling at 10% last month. He had a few successful debates since then, and now 12% of the population wants to vote for him. What's the change? You want to say 2%, are we right? It's wrong! Let's examine this. Imagine the whole population is 1000 people. 10% of them is 100. 12% is 120. What's the percentage increase? It's 100 × 20 / 100 = 20%!

Computing

One practical way to understand the function of the natural logarithm is to put in the context of compound interest. That is the interest that is calculated on both the principal and the accumulated interest. The term percent is often attributed to Latin per centum, which means by a hundred. Actually, it is wrong. We got the term from Italian per cento — for a hundred. The percent sign % evolved by the gradual contraction of those words over centuries. Eventually, cento has taken the shape of two circles separated by a horizontal line, from which the modern % symbol is derived. The history of mathematical symbols is sometimes astonishing. We encourage you to take a look at the origin of the square root symbol! The percentage tells you how number A relates to number B. A real-world example could be: there are two girls in a group of five children. What's the percentage of girls? In other words, we want to know what's the ratio of girls to all children. It's 2 out of 5, or 2/5. We call the first number (2) a numerator and the second number (5) a denominator because this is a fraction. To calculate the percentage, multiply this fraction by 100 and add a percent sign. 100 × numerator / denominator = percentage. In our example, it's 100 × 2/5 = 100 × 0.4 = 40. Forty percent of the group are girls. That's the entire procedure of converting between decimal fractions and percentages.