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Do the math: Outsmarting Stats. Holly Hacker EWA Webinar Jan. 2012. Innumeracy: Excuses, excuses. No math required by my j-school Math ignorance as badge of honor We’ve got our nerd My source will give me the numbers we need. Why learn stats?. The dreaded SAT story. Percentage change.

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slide1

Do the math: Outsmarting Stats

Holly Hacker

EWA Webinar

Jan. 2012

innumeracy excuses excuses
Innumeracy: Excuses, excuses

No math required by my j-school

Math ignorance as badge of honor

We’ve got our nerd

My source will give me the numbers we need

why learn stats
Why learn stats?

The dreaded SAT story

percentage change
Percentage change

(New – Old) Over Old

NOOO!!!

20102000

18,000 students 13,500 students

Difference: (18,000-13,500)/13,500

= + 0.33

what if it s reversed
What if it’s reversed?

20102000

13,500 students 18,000 students

Difference: (13,500-18,000)/18,000

= - 0.25

slide12
Texans paid an average of $855 a year for coverage on their homes, about 47 percent more than the U.S. average of $455, according to the study.

CORRECTIONS, CLARIFICATIONS:

The story … incorrectly compared U.S. and Texas rates. It should have said that U.S. residents pay about 47 percent less than Texans do for homeowner coverage.

beware of small bases
Beware of small bases

District A grew from 100,000 to 110,000 students – 10% increase

District B grew from 1,000 to 11,000 students – 1,000% increase

slide14

Rates

Students per teacher (turn your PER into a division sign)

Total students Total teachers

500 32

500/32 = 15.6

=15.6 students per teacher

the duh factor
The “Duh” factor

California has the most fertility clinics in the nation, with most concentrated in the Bay Area and Los Angeles, according to a new report. . .

"Fertility procedures parallel population size, and California has the most women of childbearing age of any state," said report author Dr. Victoria Wright, public health analyst at the Centers for Disease Control and Prevention.

newsroom stats
Newsroom stats

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

Median: Sort the values, then find the middle one

weighted average
Weighted average

Don’t average averages.

Teacher pay = $37,000

Janitor pay = $20,000

Principal pay = $75,000

Avg district pay = $44,000 ?

weighted average1
Weighted average

10,000 x $37,000 = $370 m

2,000 x $20,000 = $ 40 m

500 x $75,000 = $ 37.5 m

12,500 $447.5 m

$447,500,000/12,500

Avg district pay = $35,800

public opinion surveys
Public opinion surveys

Survey vs. census

A random sample is necessary

Size of the population being sampled doesn’t matter – only sample size matters

sampling error
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.

reporting poll results
Reporting poll results

Don’t report unscientific polls.

reporting poll results1
Reporting poll results

Don’t report unscientific polls.

Beware of big error margins on subgroups.

slide26

Do the math

1,007 x .45 = 453

1/√453 = .047

+/- 4.7 points

reporting poll results2
Reporting poll results

Don’t report unscientific polls.

Beware of big error margins on subgroups.

Don’t forget that a poll at best is a snapshot of now, not a predictor of the future.

standard deviation
Standard deviation

The most common measure of spread – that is, how much things are spread out or clustered around the average.

percentile
Percentile

A measure that indicates the percent of a distribution that is equal to or below it.

If you received a score of 95 on a math test and this score was greater than or equal to the scores of 88% of the students taking the test, then you would be in the 88th percentile.

correlation
Correlation

With correlation, you can look at how often two or more things happen together – that there is some direct or inverse relationship.

regression
Regression

A tool that examines the relationship between two or more variables (such as poverty and test scores).

It then allows you to make predictions based on that relationship.

numbers v words
Numbers v. words

No more than two or three numbers in a sentence or (gasp!) paragraph.

34.7 percent vs. “about a third”

24 percent vs. “one in four”

2011 vs. “last year”

numbers v words1
Numbers v. words

Avoid false precision.

numbers v words2
Numbers v. words

Avoid false precision.

Always double-check your math answers. No, better triple-check.

numbers v words3
Numbers v. words

When all the above fails, numbers belong in graphics – bar charts, tables and at last resort a bulleted break out box.