newsroom math prof. steve doig cronkite school, asu
TRANSCRIPT
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??