Math Tools for Journalists

Math Tools For Journalists Ch. 1-4
By Hannah Williams
Nov. 3, 2008

1.1 The Language of Numbers

Numerical literacy is the hallmark of a professional journalist. Journalists should do the work to make sense of numbers so that their readers do not have to. It is their aim to make numbers easily understood.

Therefore, reporters are encouraged to “interview the numbers with the same care that you interview people” (18). Furthermore, the language used to describe numerical information is highly sensitive and must be carefully selected to communicate the proper meaning.

A few important tips include the following:

-Spell out single digit numbers and fractions less than one.

-Limit the number of numbers in each paragraph to two to three, and include no more than one in the lead.

-Do the math and interpret the results for readers.

-Refer to the AP Style Manual for terms describing numbers: i.e. fewer for countable items and less than for masses, among many and between two, higher and lower temperatures, etc.

-Use Arabic numerals (0,1,2,3…) over Roman numerals (I, V, X…) except in proper names. 

1.2 Percentages

Percentages are often used in reporting. A percentage point merely means “of every hundred;” thus when calculating percentages, reporters must either move the decimal point two places to the right or multiple by 100 (either way is the same thing).

The most common percentages are calculated as follows:

-Percentage increase/decrease = [(new # - old #) / old #] x 100

-Percentage of a whole = (subgroup / whole) x 100

Business news may call for the calculating of interest. Interest can either by simple, adding value to the principal investment at a percentage of the principal each year, or compounding, adding value at a percentage of the principal plus the previously accumulated interest. 

The formulas for calculating interest are as follows:

-Annual simple interest = P x R x T where P is the principal, R is the annual interest rate and T is the time in years. *Some loan repayment plans are calculated this way.

-Monthly compounding interest = [P x (1+R)^N x R] / [(1+R)^N -1] where P is the principal, R is the annual interest rate as a decimal divided by 12 and N is the number of months. *Most credit card companies use monthly compounding interest.

-Annual balance of compounded interest = P x [1+ (R / T)]^T where P is the principal, R is the annual interest rate as a decimal and T is the time in years. *Most savings accounts us this method.

1.3 Statistics

Statistics are frequently used in news reports; however, journalists must be aware that statistics are easily manipulated and report possible manipulations.

Common statistics jargon includes the following:

-Mean = the average = sum of all items / number of items

-Median = the middle number (or average of the two middle numbers) when figures are arranged from highest to lowest

-Mode = the number appearing most frequently

-Percentile rank = the percentage of scores that fall at or below the designated score = # at or below designated score / # of figures

Interpreting the results:

-Standard deviation = how much a figure deviates from the norm

-Subtract the mean from each item in the set, square each result, compute the mean for these figures, this is the variance.

-The square root of the variance is the standard deviation

-In a standard, or bell shaped, distribution, 68 percent of the data will fall within one standard deviation on either side of the mean (higher or lower), 95 percent within two and 99 percent within three.

-Probability = “one out of X population” = # of occurrences per total population / total population

-Chance a series of events will occur = odds of first event x odds of second event x odds of third event, etc.

-Chance a series of events will occure when odds of each outcome is equal = (O)^N where O is the odds of each event and N is the number of possible outcomes

1.4 Federal Statistics

The federal government constantly publishes statistics as an indication of the country’s performance. These statistics include the unemployment rate, inflation as indicated by the Consumer Price Index, the gross domestic product, and the international trade balance. The first two can be found at the Bureau of Labor Statistics site, www.bls.gov, and the latter two can be found at the Bureau of Economic Analysis site, www.bea.gov.

The unemployment rate = unemployed people (those who have neither received pay for any work within the week prior to the survey nor completed 15 hours of unpaid work for a family enterprise) / labor force (those over the age of 16 who are employed or are actively seeking employment) x 100

Inflation as indicated by the Consumer Price Index is calculated monthly and annually and used to adjust prices to account for inflation.

-Monthly Inflation Rate = (current CPI – prior month CPI) / prior month CPI x 100

-Annual Inflation Rate = (current CPI – prior year’s CPI in same month) / prior CPI x 100

-Price Adjusting:

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

            -Cost after 1 year = Original Cost x [1+(Annual Inflation Rate/12)]¹²

GDP = measure of the value of a country’s goods and services = Consumer spending + Investor Spending + Government Spending + Exports – Imports.

GDP per Capita = GDP/Citizens

Trade Balance = the difference between exports and imports = Exports – Imports.

Problems

1.1 If a ship is moving at 18 knots, what is its speed in miles per hour?

a.     Need to know the conversion factors: 1 knot = 1 nautical mile per hour = 6076 feet per hour
and1 mph =1 mile per hour = 5280 feet per hour

b.     18 knots x 6076 feet/hour x 1 mile/5280 feet = 20.7 mph

c.      That’s the math, but you could also use the conversion calculator and it would do the math for you.

1.2 If a student takes out $50,000 in loans to pay for college, and the federal interest rate is compounded monthly at an annual rate of 1.5 percent, what is the student’s interest payment 1 year after taking out the loan (round to the cent)?

a.     Monthly compounding interest = [P x (1+R)^N x R] / [(1+R)^N -1]

b.     P = $50,000; R= .015 percent/year / 12 months/year = .00125 percent/month; N = 1 year  x 12 months/year = 12 months

c.      Monthly compounding interest = [$50,000 x (1.00125)¹² x .00125] / [(1.00125)¹² -1] = $4,200.60

1.3 If students’ scores on the editing exam are as follows, 65, 69, 72, 74, 80, 88, 88, 88, 90, 94, 94, 97, 99, 99, what is the mode?

a.     Mode = most frequent = 88

1.4 What is the unemployment rate if 150,000 people of working age seeking work are surveyed and 4,700 of them have not been paid for their work in the past week but 300 have worked 15+ hours for a family business?

a.     Unemployment Rate = Unemployed People / Labor Force x 100

b.     Unemployed People = 4,700-300 = 4,400

c.      Unemployment Rate = 4,400 / 150,000 x 100 = 2.9 percent