Math for communicators

In generalMany press releases/stories/ads

are based on numbers.Familiarity with basic math is

necessary to convey the info the numbers represent to your audience.

If you aren’t good at math, ask for help.

Online tools can be helpful.

PrecisionNumbers are preciseWe need to accurately convey

that precision in our ads, press releases, news reports

Numbers help us express clearly, concisely the amount of change, trends, etc.

PercentagesAs proportion of part to whole

- Divide the part by the whole- Move the point two spaces right- Add the word “percent”

For example- Jones got 215 of the 312 votes cast- 215 divided by 312 is .6891- Jones got 68.9 percent of the vote

PercentagesTo express change

- Determine the base number and the new number.- Find the difference by subtraction.- Divide the difference by the base.

For example:- Enrollment last year was 12,310; this year it’s 13,200.- The difference is 890.- 890 divided by 12,310 is .0722- Enrollment has grown by 7.22 percent

For exampleIn a city of 42,000 population,

6,260 residents are Hispanic. What is the city’s percentage of Hispanic residents?

Last year, 615 students graduated from the school. This year there were 589 graduates. By what percentage have graduates decreased?

Answers6,260 Hispanic residents divided

by 42,000 total population = 0.1490. Move the decimal two places to the right – 14.9 percent.

Base number is 615. Difference is 26. Divide 26 by 615 and it equals .0422. Decline is 4.2 percent.

Measures of central tendencyThe mean (average)

- Add the individual numbers- Divide the sum by the number of numbers.- The result is the mean.

For example:- Your grades on quizzes are 4.0, 3.7, 3.7, 2.7, 4.0, 3.3- The total of the six grades is 21.4- 21.4 divided by 6 = 3.56

For exampleSix people are on a bus.Their incomes are $35,000,

$79,000, $65,000, $53,000, $27,000 and $39,000.

What is their mean income?The six added together equals

$298,000. Divide that total by six, and the result is $49,666.

Measures of central tendencyThe median

- Find the middle number- If you’ve got a sample of 21 cases, the median is the number with 10 numbers greater than it and 10 numbers smaller than it.- In even-number sets, average the two in the middle.

Median income on bus$27,000; $35,000; $39,000;

$53,000; $65,000; $79,000Since the number of cases is

even, add the two middle cases and divide by 2.

$39,000 + $53,000 = $92,000. $92,000 divided by 2 equals $46,000.

Back to the busBill Gates gets on the bus.His income is $100 million.Which measure makes more

sense to provide a snapshot of who is on the bus – the mean or the median?

Median income with Gates$27,000; $35,000; $39,000;

$53,000; $65,000; $79,000; $100,000,000.

Middle number is $53,000.

If we had used the mean to express the makeup of the bus ridership, we’d say the average person on the bus earned $14,328,285.

Measures of central tendencyThe mode

- Most frequent number in a set- If each number appears once, there is no mode- If several numbers appear the most, they are all modes- Not a very practical usage.

Mills in MichiganMills are the units used by

Michigan governments to levy taxes on property.

A mill is a tax of $1 on each $1,000 of taxable value of land, buildings and business equipment and machinery.

Taxable value can be no more than half of market value.

Millage calculationsChoose a representative taxable

value, $50,000 for example.Divide it by 1,000.Multiply the result by the millage

rate.If a city levies 2.35 mills, the

owner of a home with a taxable value of $50,000 would pay $117.50 in taxes.

SurveysThe best use of survey technique

is to use a sample to represent the whole.

A sample will not be representative unless it is truly random.

In a random sample, every member of the whole group has an equal opportunity of being chosen.

Convenience samplesConvenience samples – the first

five people you meet outside the library, for example – are not representative.

Their only valid use is to generate good quotes that illustrate points of view determined by other, more valid means.

Don’t misrepresent convenience samples as random samples.

Writing with numbersCite sources for all statistics.Long lists of figures are difficult to read

in stories. Charts and graphs may be a better option.

Round off numbers usually after two places. For example: $1.35 million rather than $1,349,276.

Always double-check your math and verify statistics a source gives you.

If you don’t understand the numbers, get an explanation.

Check the mathIf given statistics, check the

math.For example – you are told the

budget is X percent larger than last year.

Do the math and check.Journalism is the discipline of

verification.PR, advertising need to be based

on sound information.

Numbers and styleWhen writing, follow the general

rules under “numerals” and other number references in the stylebook.

Spell out fractions: one-tenth, two-thirds.

Ranking – when using numbers to indicate a position, use first, second, third … ninth. Spell out larger than 10 – 10th, 11th, 23rd, etc.

Corporate namesHere, we vary from the common

AP style rules.Instead, use the company’s

official name:- Fifth Third Bank.- Century 21 Real Estate.

Minus, informal referencesUse the word minus, not the

symbol, to avoid confusion:- It was minus-21 on Thursday.- It was 21 below zero Monday.

In informal or slang references to numbers, spell out:- Thanks a million.- He was a million to one shot to win.

Questions?