Posted by: rcieri | November 3, 2008

The Abridged Math Tools for Journalists: Wickham’s Briefing (Part 1)

By Rachel Cieri

Nov. 3, 2008

Many journalists are afraid of the preciseness of numbers. They are used to the leniency of writing, but journalists commit to high standards of accuracy that requires proficiency with numbers. Even more than simply numerical literacy, journalists need to be skeptical of numbers and never assume that the person providing them has done the math correctly.

To help readers understand more easily, journalists use AP style to present numbers a certain way. All numbers in print should be in Arabic numerals, excepting one-digit numbers, numbers higher than 1 million, fractions less than one and numbers that begin a sentence. One-digit numbers can be written as numerals in ages, weights, highway designations, percentages, speeds, temperatures, times and weights. Besides using style to clarify numbers for readers, journalists try to limit the number of numbers in each paragraph, doing the math for readers and analyzing the data for the reader. Journalists should understand qualifying terms that accompany numbers, like the difference between “less” and “fewer,” and use them correctly.

Percentages are often used by journalists to help readers make sense of what a number means in context, and they are used in four different ways: percentage increase, percentage decrease, percentage of a whole and percentage points.

Percentage increase should always yield a positive number, while percentage decrease should always produce a negative number. In order to convert the answer from a decimal to a percentage, move the decimal point two places to the right. Percentage increase and decrease are calculated using the following formula:

Percentage increase or decrease = (new figure – old figure) / old figure

Example of percentage increase:

A city council budget was increased from $4,489,220 to $5,230,670. What was the percentage increase?

Old figure = $4,489,220
New figure = $5,230,670

Percentage increase = (5,230,670 – 4,489,220)/ 4,489,220
Percentage increase = 741,450/4,489,220
Percentage increase = 0.16516
Percentage increase = 16.5 percent

Calculating the percentage of a whole is fairly basic. To do this, simply divide a part by the whole and move the decimal point two places to the right. Use the following formula:

                  Percentage of a whole = part/ whole

Example of percentage of a whole:

One poll shows that 56 out of 115 Americans plans to vote for Barack Obama. What percentage of these Americans plan to vote for Obama?

Part = 56
Whole = 115

Percentage of a whole = 56/ 115
Percentage of a whole = 0.48695
Percentage of a whole = 48.7 percent

It is important to distinguish between percentages and percentage points. If a candidate only has 15 percent of the vote, and then he later gets 17 percent of the vote, his popularity has increased by two percentage points. Those two percentage points, though, are a 13.3 percent increase.

Interest is a common form of percentage use in news, especially with the financial crisis, which is centered on loans and mortgages. Interest is always displayed as a dollar amount. Use the following formula to find interest:

                  Interest = principal x rate x time

                                    Principal = amount borrowed
                                    Rate = percent charged in decimal form
                                    Time = length of period money is kept (in years)

Compounding interest becomes a more complicated matter. This involves multiplying the interest rate by the principal to find the new balance due. This is done in intervals based on a time period. For example, if a loan’s interest is compounded monthly, the balance is multiplied by the interest rate every month to find a new balance due. Interest rates are often  given in terms of years, however, so if it is compounded monthly, the interest rate must be divided by the number of months (12) to find the new balance.

To find the monthly payment for a compounding loan, it is easiest to use an advanced calculator. Use the following formula:

A = [P (1+R)N x R] / [1+RN – 1]

                  A = monthly payment
                  P = original loan amount
                  R = interest rate (in decimal form and divided by 12)
                  N = number of months

Percentages are commonly used in statistics, which are important in showing the size and scope of an event in the news. Statistics are used in survey or study results that journalists are often asked to evaluate, so it is important understand what they mean.

The three most basic and commonly used statistical measures are mean, median and mode. The mean, commonly called the “average,” is the sum of the data points divided by the number of data points. The median is the middle number in an ordered set of data. If there is an even number of data points (no middle) and the two middle data points are not the same, their mean is the median. The mode is the number that appears most often in a set of data. There can be multiple modes or no mode at all. Each of these numbers can be used to reflect an entire set of data, but it is important to choose which most accurately reflects the data in context.

Percentiles are frequently used to describe test scores. A percentile represents the percentages of scores equal to or below a specific score. For instance, a test-taker who scored in the 80th percentile knows that she scored the same as or better than 80 percent of the other test-takers. To find a percentile rank, simply divide the number of scores at or below a specified score by the total number of scores.

Science reports will usually use standard deviation to indicate how much a set of data varies from the mean. A low standard deviation results from an experiment with consistent results, which means that the research is more likely to be valid, while a high standard deviation means the opposite. In a typical data distribution 68 percent of the scores fall within one standard deviation, 95 percent within two and 99 percent within three. Standard deviation can show things like diversity of test scores or temperatures.

Journalists can use probability to show their readers how likely it is for a certain event or situation to affect them or their loved ones. Probabilities are expressed as ratios, and they are often easier for readers to understand in the form of “one out of…” Probabilities become more accurate with more factors taken into consideration, like the population a situation is likely to affect. To calculate odds for a series of events, multiply the odds of all events in the series. If the odds for all of the events is the same, the formula becomes:

                  Odds in a series when outcomes are the same= ON
O = Odds
                                    N = Number of events

Of the many organizations that report statistics, the federal government is one of the most important. It is also important to know where these numbers come from in order to analyze and use them properly. Four of its most common statistics are the unemployment rate, inflation, the gross domestic product and the international trade balance.

The monthly unemployment rate is the percentage of the labor force that is unemployed and actively seeking work. The labor force includes anyone who has or actively sought a job in the last four weeks, and being employed means doing work for pay or 15 hours of unpaid work for a family enterprise. These statistics come from a 60,000 household sample called the Current Population Survey, and they are adjusted for seasonal disparities.

Inflation is measured by the Consumer Price index, a figure that shows the amount of inflation each month for eight major product markets that were determined by a survey on spending habits. The consumer price index can be reported as an index number, which indicates how much higher inflation has risen since the base number on 1984, or it can be reported as a monthly rate determined by this formula:

                  Monthly Inflation Rate = (Current CPI – Prior Month CPI) / Prior Month CPI x 100

An annual inflation rate is also reported each month, and it is found using this formula:

Annual Inflation Rate = (Current month CPI – CPI in same month from previous year) / CPI from same month in previous year x 100

To adjust for inflation, which can be necessary when putting old prices in context, use this formula:

                  Target year value = (Starting year value / Starting year CPI) x Target year CPI

The Gross Domestic Product is the value of goods and services produced by a nation’s economy, which indicates the direction of a country’s economy by increasing or decreasing. An increasing GDP indicates a healthy economy, while a decreasing GDP shows a recession. The GDP is reported quarterly, while the rate of growth is reported annually. GDP is calculated using this formula:

GDP = Consumer spending on goods and services + investment spending + government spending + net export

The trade balance is the difference between exports and imports. If it is negative, it shows that there are more imports than exports, while a positive number means the opposite. The seven major categories of exports and imports are capital goods other than autos, travel and other private services, industrial supplies, autos and auto parts, consumer goods, food and beverages and “other.” This number is considered comprehensive because it is not extracted from sample data.

Practice Problems

1.     A college’s tuition increased from $33,250 to $34,015. What was the percentage increase?

2.     A class of students scores the following on a test: 99, 92, 90, 88, 83, 81, 79, 77, 77, 75, 74, 73, 69, 66, 64, 60, 57. In what percentile are the students who scored 77?

3.     How likely is it that a person who drives a car without a drivers’ license three time will cause an accident all three times? The odds for this happening once are 1/5.

4.     What is the monthly inflation rate for September if the CPI for September is 218.783 and the CPI for August is 219.086?


1.      2.3 percent

2.     58th percentile

3.     1 out of 125

4.     -0.13 percent



  1. Wow, Rachel. Amazing look at the use of numbers! I KNOW you gained insights you can use to be a better reporter when you processed this information so thoroughly. You did yourself a big favor by putting so much effort into this.

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