SLA Notes: Ratios, Percentages and Statistics, Oh My!
Ratios, Percentages and Statistics, Oh My!
Wednesday - June 8, 2005
7:00 - 8:30 A.M.
John Martin - St. Petersburg Times
Steve Doig - Knight Chair, Computer Aided Reporting, Kronkite School of Journalism, Arizona State University
John Martin:
Topics -
* State of newsroom math skills
"The media's sloppy use of numbers about the incidence of accidents or disease frightens people and leaves them vulnerable to journalistic hype, political demagoguery and commercial fraud." - Max Frankel, 1995, NY Times
Most-repeated definition of a journalist: a do-gooder who hates math
Contributing factors to poor math skills in newsrooms:
- curricula of journalism schools not including math/stat courses
- math anxiety
- hubris
- newsroom training, or lack thereof
- as the editors go, so goes the newsroom
Core newsroom math skills:
- Basic arithmetic
- Ability to calculate percentages and ratios
- Knowing the difference between mean and median
- Understanding margins of error
* Common math mistakes
When calculating percent change, not changing the formula if you're looking at a % increase or a % decrease
Attempting to combine heterogeneous statistics or compare percentages with different bases
Not understanding the difference between percent change and percentage points
Confusing "times more" and "times as many"
* Fundamental facts
- No percentage makes sense unless you know its base
- Understanding mean, median and mode:
mean: simple average (but can be skewed by extremes at either end of a set of values)
median: the middle values in a group
mode: the most commonly recurring value
* What reporters should do?
- Keep copy numbers-free
- Keep number of digits in a paragraph below 8
- Round off -- a lot
- Memorize common numbers on a beat
- Use devices/metaphors from everyday life
- Learn to think in ratios
- Learn from one another
* What is the role for librarians?
- News librarians: the bartenders of the newsroom - listen to reporters' frustrations
- Avoiding Chicken Little syndrome: don't be too melodramatic in demonstrating the need for math training in the newsroom
- Virtue of the Dog and Pony show: make training into a formal event
- Marketing, marketing, marketing
- You can lead a journalist to training, but you can't make 'em think
Steve Doig:
Why math? -- We make too many stupid mistakes; to be a good journalist, you must be good at math, but the good news is that it's only 6th grade math
How to do math -
* Comparing new and old values (i.e. percent change)
[(NEW / OLD) - 1] X 100
This formula covers percent increases AND decreases
Beware of base changes
Beware of small bases/figures
* Rates
number of events per some standard unit (per capita; per 100,000; etc.)
Use to compare places of different size in terms of crime rates, accident rates, etc.: # of events / population X per unit
* Consumer Price Index (CPI)
Use the CPI to convert for inflation when comparing old and new prices/income/etc.:
price now / price then = CPI now / CPI then
* Basic newsroom statistics
- Maximum, minimum and range
- Mean/average
- Median - sort the values and find the middle one
- Mode - most commonly used values
* Weighted averages: don't average averages
* Public opinion surveys
- Survey vs. census: a survey is a random sampling of a population; a census is the count of the entire population
- The size of the population doesn't matter -- only the size of the sample matters
- Sampling error: the bigger the sample, the smaller the error -- formula: 1 / square root of n, where n=sample size
* Reporting poll results
- Don't report unscientific polls
- Don't make a big deal about small differences/statistically insignificant results
- Beware of big error margins on subgroups
- Don't forget that a poll at best is a snapshot of the present, not a predictor of the future
* Estimating crowds
- Beware of the official estimate (on both sides)
- A better method:
1) Estimate the are in square feet (length X width)
2) Divide by:
-- 10 - for a loose crowd
-- 7.5 - for a tighter crowd
Try to account for turnover