Q. When reporting the cost of living, why do analysts use the phrase “excluding food and energy costs’? … (And) why do analysts, when reporting a company’s performance in dollars per share, sometimes mention “excluding a one-time charge’ …? Food and energy are major factors in one’s living expenses, just like unforeseen corporate expenses happen and are a part of (a company’s) performance.

R.J.R., Warwick, R.I.

A. Mainly it’s because the analysts want to look not only at an overall number, but also at the underlying figures, to try to spot trends and compile their own forecasts, according to two professors interviewed.

Let’s take the points you raised one at a time:

INFLATION: In general, the federal government surveys the cost of goods and services in our economy. The government generally combines the results to come up with an overall measure, an index, called the Consumer Price Index (CPI).

If the overall CPI goes up, it generally means that the overall price of goods and services has risen (inflation). But does it also mean that the price of every single item in the survey has risen, too?

No. That’s why many economists and others who analyze these figures like to look not only at the overall CPI, but also at the underlying figures.

Food and energy represent a big chunk of the CPI. But they’re not the only components, and they can also be volatile – they can go up or down sharply in any given month, said Allan Graham, assistant professor of accounting at the University of Rhode Island’s College of Business Administration in Kingston.

If the overall CPI goes up, maybe it’s because food and energy rose sharply, while other items stayed the same or even declined. So it’s helpful to know not only how the overall index did in any given month, but also how the “core” index – excluding food and energy – performed.

By looking closely at the numbers, economists and others can generally tell whether a big jump up or down in the overall index was caused solely by food and/or energy.

They can also tell whether there’s a trend in the level of prices for other items in the index, and make their own forecasts, Graham said in an interview at his office.

Yes, we all pay for food and energy, and we can’t ignore the cost of either. But the broad index does count other items, too.

A casual user of such information may be satisfied with knowing only the overall number, said Hakan Saraoglu, associate professor of finance at Bryant College in Smithfield, R.I.

But if you’re an investor making investment decisions, for example, or if you’re an employer who bases pay raises at least in part on inflation figures, you need to go beyond the overall number, Saraoglu said. “The information should be matched with the use of that information,” he said.

“If you’re looking for confirmation that life felt more expensive last month, look at the overall (CPI number),” Saraoglu said. But if you want to know whether that’s just a passing phase, “look at the core rate” of inflation, he said.

You may find it helpful to look at the detailed results of the government’s regular CPI surveys: www.bls.gov.

CORPORATE EARNINGS: Publicly held companies regularly publish some of their financial results. A company typically publishes not only the overall dollar amount of its profits (or losses) for a given period, but also the amount of profits (or losses) divided by the number of its shares – to make it easier for comparison’s sake.

One of the most important numbers is a company’s net profit or loss. In general, this figure includes the impact of certain one-time events (also known as “nonrecurring” or “extraordinary” items), such as the dollar cost of staff layoffs, plant closings or the financial impact of prevailing in a big lawsuit.

Analysts like to know that figure, of course, but they also want to know what profits (or losses) would have been not counting these one-time items.

That way, analysts can tell how a company’s basic operations are performing – when one-time items are excluded – and make their own forecasts – assuming the figures are accurate, Graham said.

Yes, these one-time items are real and they are important. But it’s also helpful to know how a company’s underlying operations – a company’s main lines of business – are performing. Analysts and others “want to know what is persistent about a company’s performance,” Graham said.

Here, too, a casual user may be interested only in one figure. But an investor needs to dig further into the numbers, to find what was excluded and why. An investor also wants to see how a company’s core operations did, Saraoglu said in an interview at his office.

You probably do this with your own household budget. It includes the cost of things you can reasonably predict, such as the monthly mortgage or rent payment, and the cost of electricity, for instance.

From time to time, however, your household budget gets hit with an “extraordinary” item, such as the cost to replace a failed hot-water heater. In analyzing your household budget over time, you need to know not only the overall figure (including the hot-water heater), but also the underlying figure (not counting the hot-water heater).

That way, you can make a reasonably accurate prediction about the future cost of regular expenses, and make some provision for one-time items, too.

Neil Downing is a Journal staff writer and author of ” The New IRAs and How to Make Them Work for You.”