While today might just be another Monday for most of us around the world, for the nearly 37 million claiming Irish ancestry (more than 8 times the current population of Ireland), for many living on the Emerald Isle and for those who might just be interested in donning some green and downing a Guinness, St. Patrick’s Day is a beloved holiday. Following in the vein of such posts as Christmas Economics and Halloween Economics, we wanted to get in on the St. Paddy’s fun, but of course with an economics twist.

St. Patrick’s Day is historically a Catholic holiday celebrating the day that St. Patrick, patron saint of Ireland, died in 461. It has since been officially recognized as a feast day by other Christian denominations and adapted as a secular holiday, generally focusing on parades, parties and Irish-related kitsch.

Wearing and displaying the color green is a big part of St. Patrick’s Day, and many cities in the U.S., Ireland, Britain and elsewhere put green dye in their biggest rivers to celebrate the holiday, a trend that began in Chicago in 1962. This year, the city dumped 130 large containers of dye into the water to achieve a glowing green effect. In recent years, governments across the world have joined the fun and will cast a green glimmer on numerous major monuments today, including the Great Wall of China, the Leaning Tower of Pisa and South Africa’s Table Mountain.

But the biggest event for many people is the celebration itself, and all of the purchases that come with it. Last year, according to the National Retail Federation, the average person in the U.S. spent $35.27 on green clothing and décor, as you can see compared with the average total spent monthly on games, hobbies and toys. Nationwide, the overall tab for the holiday grew to $4.7 billion. Much of this is due to alcohol sales, particularly things like beer and whiskey. The forecasted total for beer sales in the U.S. was upwards of $255 million.

The beer leading the pack is of course Guinness, which has been brewed in Dublin since 1759. While we already devoted a full post to Beeronomics, we did not mention that it was in fact in a Guinness brewery in 1900 that the statistical technique called the t-test was first invented. A mathematically-minded brewer named William Sealy Gosset devised a way to evaluate the effects of diverse barley varieties on the beer using a relatively small sample size. He published his findings in a paper titled “The probable error of a mean”, thereby forever altering statistical methodology and solving a brewing conundrum for Guinness.

So, if you’re among those waiting in line to order a pint of this famous stout tonight, remember that it’s statistics that allowed for its global success. And with that, happy St. Patrick’s Day!