Math Tools Pt. 1 – Numbers, Percentages and Statistics

By Patrick Larsen

Ch. 1 – The Language of Numbers

According to Kathleen Woodruff Wickham, author of Math Tools for Journalists, it’s the precision of numbers that scares most journalists. Despite that, they are present in every newspaper without fail. At the end of the day, numbers are essential to good journalism – they provide solid context and framing that even good writing may not be able to provide. It’s important for journalists to understand basic style rules from the start, such as knowing when to spell numbers out as opposed to using numerals.

Ch. 2 – Percentages

Percentages can be used to great affect when attempting to show how meaningful certain numbers are. There are several techniques that journalists should understand in finding percentages, according to Wickham.

Percentage of a whole is the most basic type of percentage used – all you do is take a numerical value that is part of a whole and divide it by the value of the whole. From there, simply move your decimal point two spaces to the right.

Next is calculating percentage increase/decrease. This is used when a change in values within a whole has occurred over time and you need an understandable representation of that change. To do this, take the new figure and subtract the old figure from it. Then take the solution of that equation and divide it by the whole – this will give you the percentage change.

Finding percentage points also has to do with increase and decrease, but it’s a little more abstract and it starts from before and after percentage values. To find them, start by subtracting the original percentage value from the new percentage value – this gives you change expressed in percentage points. From there, you can divide the change value by the original value to see the percentage change.

Calculating interest is also of interest to us. Ha. To find this, you’ll need a few values: money borrowed, interest rate and the length of time it’s borrowed for. All you do is multiply those values together.

Compound interest is the next and more complicated step.  Instead of interest that grows at a constant rate, the rate of compound interest is built over time by have interest charged on interest.  To calculate this, you will need values for the monthly payment (A), the original loan amount (P), the interest rate (R) and the total number of months interest is being charged for (N).  Also note that the interest rate should be expressed as a decimal and divided by 12.  The formula is as follows:

A = [P x (1 + R)^N x R] ÷ [(1 + R)^N – 1]

Ch. 3 – Statistics

According to Wickham, statistics are the second most common number journalists use, after percentages.

“Having a basic understanding of statistics and the role played by the manipulation of numbers is an important element in a journalist’s toolbox.”

First, we need to understand mean, or average.  It’s very straightforward.  All you have to do to find mean is add together all of the values in a data set and divide by the number of values.

Next is median, and it’s just about as easy.  All you have to is order each value in a data set from lowest to highest and find the center value.  If there is an even number of data points, just add those two values together and divide by two.

Mode is the next statistical value we can find.  It simply refers to which values in a data set occur the most often, and all you have to do is count the number of times each value appears.  If multiple values are tied for most appearances, they are all the mode.  If none do, there is no mode.

These are all measures of average that serve their own purpose, but what else is there?  For starters, we have percentile – a way of measuring where a score falls in comparison to others.  This is done by dividing the rank of a value in an ordered data set by the total number of values in the set.

Next, we have standard deviation, which is used to express how much a figure differs from what is normal.  The formula is represented in the following image, copied from Wickham’s work:

Probability is also useful to journalists.  It’s also very easy to calculate.  All you have to do is (using deaths from a certain disease as an example) take the number of people affected and divide that by whatever population you are considering.  That number will probably be difficult to understand, so just divide one by the value and treat it as an “every one in x” value.

Ch. 4 – Federal Statistics

The government doesn’t always do the public favors with the numbers it releases.  Oftentimes the statistics are difficult to understand or unclear in purpose.  That means the onus falls on journalists to provide accurate and valuable versions of that data.

Unemployment is a federal statistic that many people hear about almost daily.  It’s consistently in the news, so we have to be sure to know how to properly calculate it.  Fortunately, it’s a pretty easy equation:  all you have to do is take the number of unemployed people in the labor force and divide that by the total labor force and multiply it by 100.  Essentially, the labor force refers to the total number of people above the age of 16 who is employed or looking for employment.

Another frequently used term in reporting is inflation, which is measured using the Consumer Price Index.  When finding this for a month, use this equation:

(Current CPI – Prior Month’s CPI) ÷ Prior Month’s CPI x 100

Annual inflation rate can be found by changing “Prior Month’s CPI” in the last equation to the CPI from one year earlier.

It may also be necessary or helpful to adjust for inflation in certain stories.  For example, if you need to change a past value into current dollars, you would divide the original value by the CPI for its year.  Then multiply that by the CPI for the present day.

Gross Domestic Product also comes up quite a bit.  This can be found simply by adding consumer spending on goods and services, investment spending, government spending and net exports.  GDP is important because it can tell a lot about how a nation’s economy is changing.

Finally, net exports – the last piece of the GDP equation – simply refers to number of exports minus the number of imports.  It is also commonly known as trade balance and is a helpful perspective on an economy.


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