Math for Reporters and Copy Editors

By Wanda Cash
The Baytown (Texas) Sun
Presented at the 2001 ASNE High School Journalism Institute at the University of Texas at Austin.

Reporters generally know how to write. Copy editors generally know grammar, spelling and punctuation. But people in neither group got their necessarily jobs because they were good at math. Here is a primer for those editors and reporters facing deadline math.

  • To figure a percent, divide the small number by the larger and multiply by 100. For instance:
    To determine what percent 15 is of 60, divide 15 by 60 (15/60=.25), then multiply by 100 to get 25 percent.
  • Often you’ll see the phrasing “one out of 80” or words to that effect. To figure this out, divide the larger number by the smaller number. For instance: To figure the “one out of …” for 15 out of 60, divide 60 by 15 (60/15=4). So it is one out of four.
  • Beware of phrasing such as “five times more than” and “five times as much.” Consider these examples:
    • If a building is worth $10,000 and you paid “five times more than it’s worth,” then you would pay $60,000 (the $10,000 plus five times $10,000.)
    • If a building is worth $10,000 and you paid “five times as much as it’s worth,” you would pay $50,000.
  • Use the phrasing of “an increase to 2 percent from 1 percent.” To say “… from 1 percent to 2 percent” might be misinterpreted at “anywhere between 1 percent and 2 percent.”
  • Beware of the phrasings “more than” and “less than.” Check them out. For instance, let’s say a story says, “More than 17 percent said they preferred chocolate chips.” You work out the arithmetic and get .1699999. That is not more than 17 percent.
  • To figure the percent of change between two numbers, find the difference between the two numbers and divide by the original.
    • For instance, if the increase is from 1 to 2, you would figure it like this:
      • The difference between the numbers is 1.
      • Because you started with 1, it is the original figure.
      • So, 1/1=1.00 (multiply by 100 and you have 100 percent)
      • That’s to say, going from 1 to 2 is a 100 percent increase.
    • The percent of increase often can be more than 100 percent. If the hotel tax goes from $3 to $9, then:
      • The difference between the numbers is 6.
      • 3 is the original, so divide 6/3=2.
      • Multiply by 100 to get an increase of 200 percent.
    • For a decrease, it works the same. From 2 to 1, you get 1 (difference) divided by 2 (original)=.50 or a 50 percent decrease.
  • Any breakdown of percentages should add up to 100. If not, you have to check it out. One number could be wrong, or a category could be missing. Don’t guess!
  • If you calculate a group of percentages, beware of “rounding errors.”
    • Let’s say you figure a percentage and get .45603. This should be rounded to 46 percent, or if you decide to be more precise, 45.6.
    • If you get an answer of .453989, round it to 45 percent (or 45.4 percent).
  • When you have all the percentages figured, they should add up to 100 percent. If they don’t you can adjust one or two of the numbers.
    If the difference from 100 percent is large (more than 1 percent), then you might consider carrying the percentages out to a decimal place (45.6 percent).
  • Beware of the difference between “a 10 percent increase” and “an increase of 10 percentage points.” If government spending for space programs increases to 20 percent of the budget from 10 percent, that’s an increase of 10 percentage points, or an increase of 100 percent.