Newsroom math Prof. Steve Doig Cronkite 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 6 th 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 = % of 45 = * 45 = 58.7

Comparing X and Y X is what percent of Y? X is X/Y of Y Then multiply X/Y by 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) = – 1 = = -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 = = -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 = 1/10 th 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, / = … * = 16.5 murders per 100,000 population

Consumer Price Index Used to correct for inflation Get the CPI at 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: € Janitor average: € Principal average: € Simple average: €35 667

Weighted average (continued) Teachers: x € = €270,0m Janitors: x € = € 30,0m Principals: 500 x € = € 32,5m Sum: people €332,5 million Weighted average: € (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=1001 /  100 = 1/10 = +/-10 pts. N=4001 /  400 = 1/20 = +/- 5 pts. N=9001 /  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 (4 th edition)”, by Phil Meyer “Innumeracy”, by John Allen Paulos “A Mathematician Reads the Newspaper,” by John Allen Paulos

Preguntas??