Confessions of a Mad Librarian

* 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