Math For Journalists

For Editing Class / By Norman P. Lewis


Percent

  1. “Percent” is composed of “per” and “cent,” the latter referring to 100. So “percent” is the frequency of something per 100 units.

  2. Percent is the portion divided by the whole X 100. In a class of four males and 16 females, the portion that is male would be 4 ÷ 20 = 0.20 X 100 = 20 percent.

  3. Percent change is calculated by computing the difference (change) and dividing it by the old (not necessarily smaller) number. The percent change for a class that grows from 18 students to 20 students is 2 ÷ 18 = 0.111, or an 11.1 percent gain. The percent change for a class that shrinks from 20 students to 18 is 2 ÷ 20 = 0.10, or a 10 percent decline.

  4. Changes in percentages are calculated the same way. About 20 years ago, UF was about 46 percent female. Now it is about 53 percent female. The portion of the student body that is female has grown by 7 ÷ 46 = 0.152, or 15.2 percent.

  5. In the example above, be careful not to confuse percent with percentage points.

RIGHT: The portion of the student body that is female has grown by 15.2 percent.

RIGHT: The portion of the student body that is female has grown by 7 percentage points.

WRONG: The portion of the student body that is female has grown by 7 percent.

Average (mean, median)

  1. Mean: add the numbers in a series and divide by the number of numerals. For example, the mean for the numbers 2, 8 and 11 is the sum (2 + 8 + 11 = 21) divided by the number of numerals (3), or 21 ÷ 3 = 7.

  2. Median: the center number in a series arranged in sequence. For example, the median of 2, 8 and 11 is 8. If there is an even number of numerals, the median is the mean of the two middle numerals.

  3. Mean is used most frequently, so much so that it is usually considered synonymous with “average.” It is commonly used with data sets that do not have large extremes, such as grade point averages, gasoline prices and most sports statistics.

  4. Median is the better statistics to use with data sets with wide variation, such as household income, housing prices and intelligence scores, in which big figures at one end (a CEO’s salary, a billionaire’s mansion, Albert Einstein) skew the numbers.

Comparisons

  1. Rather than use large numbers that lack context, use comparisons readers can grasp. Be sure the terms are equal: convert yards and feet to the same measurement before making comparisons. One square yard (1 yard = 3 feet) is 9 square feet (3 X 3 = 9).

  2. Area: The size of a large store can be compared to the number of football fields or houses that would fit inside. One cubic yard is 27 cubic feet (3 X 3 X 3 = 27).

  3. Volume: The amount of material excavated at a construction site, for example, can be converted to the number of dump trucks or swimming pools it would fill.

  4. Per capita comparisons are useful for large numbers. For example, President Bush’s proposed 2008 budget would expand the national debt by $1,300 for every man, woman and child in the United States, or $5,200 for a family of four.

  5. Time comparisons are useful for ongoing events. For example, the direct cost of the Iraq war is $4,700 a minute. A baby is born every eight seconds in the United States.

  6. Per capita comparisons should be adjusted to “per 100,000 residents” comparisons for numbers such as crime. For example, to compare crime rates between Tampa and St. Petersburg, crime should be divided by the (population divided by 100,000), to compare apples to apples.

  7. When comparing numbers, consider differences in base numbers. For example, at UF, 14 percent of male undergrads are in fraternities while 12 percent of female undergrads are in sororities. But that doesn’t mean there are more males than females in the Greek system, because in raw numbers, females exceed males at UF.

Probability

  1. Calculate the probability of an event by dividing the number of chances by the entire set. For example, the probability of pulling an ace out of a deck of cards is 4 ÷ 52 = 0.077 = 7.7 percent. You can also simplify 4 in 52 to read “1 in 13.”

  2. Probability can be expressed as a ratio, a figure or a percent. For example, the probability that a baby will be a girl is 1 in 2, or 0.5, or 50 percent.

  3. Calculate the probability of multiple events expressed by and by multiplying the odds. The probability a couple planning on two children will have two boys (one boy and one boy) is 1 in 2 X 1 in 2, or 1 in 4. Or, it is 0.5 X 0.5 = 0.25 = 25 percent.

  4. Calculate the probability of multiple events expressed by or by adding the odds. For example, the probability a batter with a .250 average will get a hit in one of four at-bats (first at-bat or second at-bat or third at-bat or fourth at-bat) is 0.25 + 0.25 + 0.25 + 0.25 = 1.00 = 100 percent.