NEW YORK - Tens of millions of older Americans are about to get what may be the biggest raise of their lifetimes.

On Thursday, the U.S. government is set to announce how big a percentage increase Social Security beneficiaries will see in monthly payments this upcoming year. It’s virtually certain to be the largest in four decades. It’s all part of an annual ritual where Washington adjusts Social Security benefits to keep up with inflation, or at least with one narrow measure of it.

Plenty of controversy accompanies the move, known as a cost-of-living adjustment or COLA. Critics say the data the government uses to set the increase doesn’t reflect what older Americans are actually spending, and thus the inflation they’re actually feeling. The increase also is one-size- fits-all, which means beneficiaries get the same raise regardless of where they live or how big a nest egg they may have.

Here’s a look at what’s happening:

What’s the big deal?

The U.S. government is about to announce an increase to how much the more than 65 million Social Security beneficiaries will get every month. Some estimates say the boost may be as big as 9 percent.

What do beneficiaries have to do to get it?

Nothing.

Will this be the biggest increase ever?

No, but it’s likely the heftiest in 40 years, which is longer than the vast majority of Social Security beneficiaries have been getting payments. In 1981, the increase was 11.2 percent.

When will the bigger payments begin?

January. They’re also permanent, and they compound. That means the following year’s percentage increase, whatever it ends up being, will be on top of the new, larger payment beneficiaries get after this most recent raise.

What’s the typical increase?

Since 2000, it’s averaged 2.3 percent as inflation remained remarkably tame through all kinds of economic swings. During some of the toughest years in that stretch, the bigger worry for the economy was actually that inflation was running too low.

Since the 2008 financial crisis, the U.S. government has announced zero increases to Social Security benefits three times because inflation was so weak.

So the increase is to make up for inflation?

That’s the intent. As Americans have become painfully aware over the past year, each $1 doesn’t go as far at the grocery store as it used to.

How is the size of the increase set?

It’s tied to a measure of inflation called the CPI-W index, which tracks what kinds of prices are being paid by urban wage earners and clerical workers.

More specifically, the increase is based on how much the CPI-W increases from the summer of one year to the next.

Is that the inflation measure everyone follows?

No. People generally pay more attention to a much broader measure of inflation, the CPI-U index, which covers all urban consumers. That covers 93 percent of the total U.S. population.

The CPI-W, meanwhile, covers only about 29 percent of the U.S. population. It has been around longer than the CPI-U, hich the government began compiling only after the legislation that required Social Security’s annual increases be linked to inflation.

Is that weird?

Yes, and some critics have argued for years that Social Security should change to a different measure, one that’s pegged to older people in particular.

Another experimental index, called CPI-E, is supposed to offer a better re- flection of how Americans aged 62 and above spend their money. It has historically shown higher rates of inflation for older Americans than the CPI-U or CPI-W, but it has not taken hold. Neither have other measures compiled by organizations outside the government that hope to show how inflation affects older Americans specifically.

How is the size set for social security benefits?

Through a complicated formula that takes into account several factors, includ- ing how much a worker made in their 35 highest-earning years. Generally, those who made more money and those who wait longer to start getting Social Security get larger benefits, up to a point.

This year, the maximum allowed benefit for someone who retired at full retirement age is $3,345 monthly.

Will rich people get the same boost in social security?

Yes. Everyone gets the same percent- age increase, whether they have millions of dollars in retirement savings or are just scraping by.