Newsroom math

Prof. Steve DoigCronkite School, ASU

Journalists hate math Definition of journalist: A do-gooder who hates

math. “Word person, not a numbers person.” 1936 JQ article noting habitual numerical errors

in newspapers Japanese 6th graders more accurate on math test

than applicants to Columbia’s Graduate School of Journalism

20% of journalists got more than half wrong on 25-question “math competency test” (Maier)

18% of 5,100 stories examined by Phil Meyer had math errors

Bad examples abound

Paulos: 300% decrease in murders Detroit Free Press (2006): Compared

ACS to Census data to get false drop in median income

KC Star (2000): Priests dying of AIDS at 4 times the rate of all Americans

Delaware ZIP Code of infant death NYT: 51% of women without spouses

Common problems

Numbers that don’t add up Making the reader do the math Failure to ask “Does this make

sense?” Over-precision Ignoring sampling error margins Implying that correlation equals

causation

Dangers of journalistic innumeracy

Misleads math-challenged readers/viewers

Hurts credibility among math-capable readers/viewers

Leads to charges of bias, even when cause is ignorance

Makes reporters vulnerable to being used for the agendas of others

The bad news…

To be a good journalist, you

MUST be able to do math

The good news! …it’s grade school math! None of this stuff:

Calculus Geometry proofs Base-12 Venn diagrams Ballistics Etc….

My office bookcase

Sarah Cohen’s tips

Keep the digits in a paragraph below 8

Memorize common numbers on your beat

Round off – a lot Learn to think in ratios Envision your dream number, then

calculate it

Newsroom math crib sheet

Comparing numbers

Difference Percent Percent difference Percentage change Millage Per capita

Difference

Okay, I won’t insult you….

Percents

To get X% of Y: Turn X% into a decimal, then multiply

by Y 20% of 90 = 0.2 * 90 = 18 130.5% of 45 = 1.305 * 45 = 58.7

Comparing X and Y X is what percent of Y? X is X/Y of Y Then multiply X/Y by 100 5 and 8:

5 is 5/8 of 8 5 is .625 of 8 (or 62.5%)

8 and 5 8 is 8/5 of 5 8 is 1.60 of 5 (or 160%)

Comparing NEW and OLD Percentage change! (NEW/OLD – 1) Or: (new – old)/old €8 million this year, €5 million last year

(8/5 – 1) = 1.6 – 1 = 0.6 = 60%, So the budget has increased 60%

€5 million this year, €8 million last year (5/8 – 1) = 0.625 – 1 = - 0.375 = -37.5% So the budget has decreased 37.5%

Remember PEMDAS!

Order of algebraic operations: Parentheses Exponents Multiplication Division Addition Subtraction

Compare X and Y (% difference)

X is (X/Y – 1) MORE/LESS than Y Use MORE THAN if the answer is

positive Use LESS THAN if the answer is negative 8 & 5: 8/5 –1 = 1.6 – 1 = 0.6 = 60%, so

8 is 60% more than 5 5 & 8: 5/8 –1 = .625 – 1 = -0.375 = -

37.5%, so 5 is 37.5% less than 8

Beware of base changes

Newsroom budget of €1 million grows by 10% one year to €1.1 million!

Next year, recession, so boss has to cut 10% from budget

Result: €1.1 million – 10% of €1.1 million = €990,000

Beware of small bases Easy to get big percentage change

when you start with small values Population 2000: 1,000

Population 2010: 1,500 Percentage change: +50%

Population 2000: 1,000,000 Population 2010: 1,100,000 Percentage change: +10%

Property tax millage

Mill = €0.001 = 1/10th of a cent per €1

Change “mills per dollar valuation” into “euros per €1,000 valuation”

Calculate tax based on “typical” value, like a €100,000 home

Example: Tax rate of 8 mills €8 per €1,000, or tax of €800

Rates

Number of events per some standard unit (per capita, per 100,000, etc.)

Crime rates, accident rates, etc. RATE = (EVENTS / POPULATION ) *

(“PER” Unit) Use to compare places of different

size

Calculating rates RATE = (EVENTS / POPULATION ) *

(“PER” Unit) If there were 320 murders in a

population of 1,937,086, what is the murder rate per 100,000

320 / 1937086 = 0.0001652… 0.0001652 * 100000 = 16.5

murders per 100,000 population

Consumer Price Index

Used to correct for inflation Get the CPI at http://www.ine.pt

Price Now = CPI Now

Price Then CPI Then

Using the CPI CPI in 2010 = 109 (base year 2005) CPI in 1990 was 52,9 Gasoline in 1990 was €0,68 per liter X / 0,68€ = 109 / 52,9 X = (109 / 52,9) * 0,68€ X = 2,06 * 0,68€ = 1,40€ Gas in 1990 cost the equivalent of

€1,40 per liter in 2010 euros

Newsroom Statistics

Mean (Average): Add the values, then divide by number of values

Median: Sort the values, then find the middle one

Mode (rarely used): The most common value

American baseball salaries

in 1994 strike year

Mean (average): $1.2 million Median: $350,000 Mode: $100,000

Weighted average

Don’t average averages Example:

Teacher average: €27 000 Janitor average: €15 000 Principal average: €65 000 Simple average: €35 667

Weighted average (continued) Teachers: 10 000 x €27 000 = €270,0m Janitors: 2 000 x €15 000 = € 30,0m Principals: 500 x €65 000 = € 32,5m

Sum: 12 500 people €332,5 million

Weighted average: €26 600 (not €35 667)

Public opinion surveys

Census vs. survey A random sample is necessary Size of the population being

sampled doesn’t matter, only sample size matters

Sampling error Rule: The bigger the sample, the smaller the

error Sampling error = 1/N

N=100 1 / 100 = 1/10 = +/-10 pts.

N=400 1 / 400 = 1/20 = +/- 5 pts.

N=900 1 / 900 = 1/30 = +/- 3.3 pts

Other sources of error

Estimating crowds

Beware the “official” estimate Better method:

Estimate the area in sq meters (L x W)

1 person/meter in a loose crowd Divide by 0,75 for a tighter crowd Account for turnover?

Newsroom math bibliography

“Numbers in the Newsroom”, by Sarah Cohen, IRE

“Precision Journalism (4th edition)”, by Phil Meyer

“Innumeracy”, by John Allen Paulos “A Mathematician Reads the

Newspaper,” by John Allen Paulos

Preguntas??