New York City is constantly changing.

Whether it's the crime rate, the urban landscape, or the Mets making it to the World Series, the island is full of surprises.

But one thing has stayed rock-solid. It's called the Pizza Principle.

Here's the theory: Since the 1960s, the price of a slice of pizza has risen in tandem with the cost of a subway ride with startling similarity.

When the Pizza Principle was first proposed in 1980 by patent attorney Eric M. Bram in The New York Times, Bram noted that pizza was on the rise: a bad omen for subway fare.

"It is impossible for any discerning New Yorker to find a decent slice of pizza for less than 60 cents," he said. "The 50-cent fare was doomed."

In the 30 years since Bram proposed the idea, it's been vetted, challenged, re-validated, and upheld by suspicious dollar-slice lovers and bored statisticians alike.

Even Bram, who first noticed the trend when fares and slices each cost 15 cents apiece, wondered whether the 2003 shift from subway tokens to variable-fare MetroCards would disrupt it. Others have pulled survey data on slice prices to claim the principle is patently dead.

So please, allow Jared Lander to clear the air.

In 2014, Lander — an adjunct statistics professor at Columbia University — performed what is probably the most comprehensive analysis of subway fares and pizza slices from around New York City.

He pulled prices from more than 1,800 pizza joints around New York that were listed on MenuPages, including the outer boroughs of Brooklyn and Queens. (There was no data on Staten Island or the Bronx.)

After he cleaned up the data and isolated the prices of slices labeled plain, cheese, or regular, he found that "the mean price was $2.33 with a standard deviation of $0.52 and a median price of $2.45," he writes. "The base subway fare is $2.50 but is actually $2.38 after the 5% bonus for putting at least $5 on a MetroCard."

Since March of this year, a single subway ride has cost $2.75. With an 11% discount if you buy a $5.50 MetroCard, or the cost of two rides, the adjusted fare is $2.45 — the exact same as the median price Lander found in his analysis.

In other words, the trend is still holding strong.

Economics is no stranger to food-based metrics.

The so-called "Big Mac Index," developed by The Economist in 1986, has for years helped analysts arrive at a loose estimation of the world's economic state. It's based on the theory of purchasing-power-parity (PPP), a measure of how far a dollar goes in different locations.

For example, the Economist explains that the average price of a Big Mac in the US in July 2015 was $4.79. In China, after account for market exchange rates, it was only $2.74.

So the Big Mac Index says that the yuan was undervalued by 43% at that time.

The quaint Pizza Principle may never leave the confines of New York City, and that's OK. New Yorkers have always marched to the beat of their own drum.