# Matematika pro novináře

Newsroom Math Crib Sheet

By Prof. Steve Doig, Arizona State University

To convert a fraction into a decimal:

• Divide the top number by the bottom number
• Examples: 5/8 = 0.625           17/64 = 0.265…

To convert a decimal into a percentage:

• Multiply by 100 (or simply move the decimal two places to the RIGHT)
• Examples: 0.658 = 65.8%           1.255 = 125.5%

To turn a percentage into a decimal:

• Divide by 100 (or simply move the decimal two places to the LEFT)
• Examples: 43.7% = 0.437           148.2% = 1.482

To get X % of Y:

• Turn X % into a decimal, then multiply it by Y
• Example: 20 % of 90 = 0.20 * 90 = 18           130.5% of 45 = 1.305 * 45 = 58.7…

To compare X and Y using percentages (X is what percent of Y?):

• X is (X/Y * 100) percent of Y
• Example: 5 and 8: 5/8 = .625 = 62.5%, so 5 is 62.5% of 8
• Example: 8 and 5: 8/5 = 1.6 = 160%, so 8 is 160% of 5

To compare X and Y using percentage differences:

• X is ((X/Y –1) * 100) MORE/LESS than Y
• Use MORE THAN if the answer is positive, and LESS THAN if it’s negative
• Example: 5 and 8: 5/8 –1 = .625 – 1 = -0.375 = -37.5%, so 5 is 37.5% less than 8
• Example: 8 and 5: 8/5 –1 = 1.6 – 1 = .6 = 60%, so 8 is 60% more than 5

To compare a NEW number with an OLD number using percentage change:

• NEW has increased/decreased ((NEW/OLD –1) * 100) percent since OLD
• Use INCREASED if the answer is positive, and DECREASED if it’s negative
• Example: This year’s \$8 million budget is a 60% increase over last year’s \$5 million budget.
• Example: This year’s \$5 million budget is a 37.5% decrease from last year’s \$8 million budget.

To calculate rates (the number of events per some standard unit):

• Do this to account for different size populations
• RATE = (EVENTS / POPULATION ) * (“PER” Unit)
• Example Problem: If there were 320 murders in a population of 1,937,086, what is the murder rate per 100,000?
• First, divide the 320 murders by 1937086 = 0.0001652…
• Now multiply 0.0001652… by 100,000 = 16.5 murders per 100,000 population

To calculate the effect of inflation using the Consumer Price Index (CPI):
Price Now         CPI Now
————–   =   ————
Price Then       CPI Then

• With this formula, all you need is any three of the numbers to calculate the fourth.
• Example: CPI now = 213.2; CPI in 1965 was 30.8; price of gas in 1965 was \$0.30 per gallon.

X / 0.30 =213.2 / 30.8
X = (213.2 / 30.8) * 0.30 = 6.92 * 0.30 = gas in 1965 cost the equivalent of \$2.08 per gallon

Newsroom statistics:

• Mean (average): Add the numbers, then divide by how many numbers there are
• Median: Sort the numbers in order, then find the middle value
• Sampling error margin: 1/√N (example: sample of 625: 1/√625 = 1/25 = 0.04 = +/- 4 points)

Crowd estimating:

• Calculate area in square feet (length X width)
• Divide by 10 for a loose crowd (people are at arm’s length)
• Divide by 7.5 for a tight crowd (people are more shoulder to shoulder)

pdf ke stažení zde: Newsroom-Math-Crib-Sheet-Steve-Doig