Math for Journalists

(& other numerically challenged COM

majors)

Berry’s budget for computer software jumped from $5,500 to $7,300. By what percent did it

increase?

Percent difference = N/O – 1(New number/ Old number -1)

(7,300/5,500) = 1.327

Subtract 1, and you get .327

To convert .327 to a percent,

Move the decimal two places to the right:

.327 = 32.7%

Comparing apples and oranges

• Rates (percent)

• Index numbers (CPI)

• Percentile rankings (SAT v. ACT)

• Standardized scores (standard deviation units)

Percent is a rate per hundred

• First, convert the fraction to a decimal by division.

• ¾ = .75• To get a rate per hundred, multiply by

100• .75 * 100 = 75• Another word for “rate per hundred” is

“percent.”

Other rates

• To get a rate per thousand, multiply by 1,000.

• .75 * 1000 = 750

• To get a rate per hundred thousand, multiply by 100,000

• .75 * 100,000 = 75,000

Example

• A military unit of 41,000 persons had 49 suicides over a 10-year period. Its 10-year suicide rate was:

• .1195 percent• 1.195 per thousand• 119.5 per hundred thousand• Choose a rate that yields whole

numbers

A gallon of paint covers 400 square feet. How many gallons to coat one

side of a fence that is 7 feet high and 300 feet long?

First figure the area: 7 * 300 = 2,100 square feet

A gallon does 400 square feet.

How many 400s in 2,100?

2,100/400 = 5.25

The police force has 693 officers, or 5 percent more than it had last year. How many did it have last year?

What number when multiplied by 1.05 yields 693?

Answer: 693/1.05 >> 660

A home valuation is $127,500,and the tax bill is $1,466.25. What is the rate per $100?(this is how taxes are computed)

How many 100s in $127,500? 1,275

What number multiplied by 1,275 yields 1,466.25?

1,466.25/1,275 = $1.15

If algebra makes it easier …

1,275 * X = 1,466.25

X = 1.466.25/1.275

X = 1.15

307-C Church Street:

The pizza problem

We have ¾ of a pizza.

How to divide 4 ways?

3/4 = 6/8 = 12/16

We need 16 slices so that each roommate gets 3/16 of the pizza.

Order-of-magnitude checks

The coach expressed concern because .05% of his team failed a drug screening. Should he be concerned?

Drug use by athletes

What the numbers mean:

5 percent = 5 out of 100 or 1 in 20

.5 percent = 5 out of 1,000 or 1 in 200

.05 percent = 5 out of 10,000 or 1 in 2,000

RALEIGH, N.C. (AP) - American history is being preserved at the North Carolina birthplaces of two of the nation's early presidents, James K. Polk (1845-49) and Andrew Johnson (1865-69).

Polk pushed an expansionist vision during his term, adding about 500 million square miles to the Western United States.

What? Think!The distance from New York to Los Angeles is about 3,000 miles.

A square with 3,000 miles on each side would contain 3,000 * 3,000 or 9 million square miles.

Therefore, the entire continental USA could easily fit inside 9 million square miles.

RALEIGH, N.C. (AP) - In an Aug. 17 story about presidential birthplaces, the Associated Press erroneously reported the amount of territory added to the United States during the presidency of James K. Polk. The correct figure is 1.2 million square miles, not 500 million square miles.

Doh!

Anything wrong here?

Average gas price likely to surpass $4

Fuel could hit record as soon as this weekend

The record is …

In nominal dollars: $4.05

In constant dollars: $5.65

So, no record will be broken this weekend.

Frozen box office take in 2013-14: $384.1 million US ($596.3 million foreign)

Gone with the Wind (1939): $199 million

What’s Frozen in constant dollars? Did Frozen in fact set a record?

Consumer Price Index

• 1939 = 13.9

• 2014 = 235.8

13.9 = $199 million

235.8 x

X = (235.8 * 199m) / 13.9

http://www.bls.gov/cpi/home.htm

$199 million in 1939 had the same buying power as $3.4 billion in 2014.

Frozen’s gross is NOT a record, even after adding foreign. It’s in 110th place.

How to write with numbers

First rule:

no more than two numbers in a paragraph

Example

Revenue at the largest generator of electricity in the USA soared to $61.3 billion in 2010 from $13.7 billion in 2009. Yet to get that much more revenue, the company added 5% more workers to 27,726 and assets declined 13% to to $47.3 billion from $54.5 billion.

--USA Today

Whaaaaa?!?!

Second rule:

if you use two numbers in a paragraph, it’s better if they can be compared with each other.

How to write with numbers

Example

As of Tuesday, Boston had a season total of 78.1 inches of snow. The average annual total for Boston is about 42 inches.

--USA TODAY

Third rule: don’t use more significant digits than you need.

• If 22.34% of the public supports Obama, report it as 22%, or even “roughly one in five.”

• Remember: polls are never actually accurate to fractions of a percentage point.

How to write with numbers

On the other hand …

Felony theft is defined as taking goods worth $50 or more. Let’s say a shoplifter steals a sweater priced at $49.99. We’re not going to round up to a felony.

Writing with numbers

• Express the concept in words first.

• Then back it up with the numbers.

• Show the numbers as part of a system.

Example

A new study finds what may seem obvious to even casual observers: Many National Football League players are obese.

--USA TODAY

Five paragraphs later …

• 56% of players qualify as obese, which is roughly 30 or more pounds over a healthy weight.

• About 26% of football players qualify as severely obese, and 3% are morbidly obese.

Writing with numbers

• Choose your typical value carefully

• Mean = the value each case would have if each case were the same (and the total was constant). Much like an average.

• Median = the value of the middle case.

• Mode = the most frequent value.

Where $$ is involved, the mean can be misleading The tax relief is for everyone who pays income taxes – and it will help the economy immediately: 92 million Americans will keep, this year, an average of almost $1,000 more of their own money.

-- Barack Obama, State of the Union Address

Income Percent of returns

Tax saving in dollars

< 10K 19.2 5

10-20 17.5 63

20-30 13.9 204

30-40 10.1 351

More than 80% receive less than $1,000

Outliers: Graduates of what UNC academic department had

the highest mean salary in 1986?

• School of medicine

• School of journalism

• School of law

• Geography department

• School of business administration

Michael Jordan, who graduated from UNC in 1986, majored in geography!

Math for Journalists (& other numerically challenged COM majors)