Welcome to the home of the IRS's master computers

The prolonged search for the missing rexeipt, the hours spend decophering convoluted instructions, the last-minute loan for the amount you owe. The result of all those tax-season rituals ends up here in a building almost hidden among the rolling hills and apple orchards of West Virginia.

The Internal Revenue Service's national Computer Center, at Martinsburg, W. Va., is the final destination for the tax information that citizens so laboriously calculated in April. This week, computer tapes filled with data from taxpayers who filed returns on the April 16 deadline will begin arriving at the Washington, D.C., airport. Then the tapes will be trucked 84 miles up the road to Martinsburg.

Three computer systems whirring around the clock in air-conditioned comfort here will make or strongly influence the following key decisions the IRS will reach on your 1983 tax return:

* Does the information on your return match what employers and banks said you earned?

* Will you be asked to send more money?

* Will your refund be approved?

* Will the government bill you for unpaid child-support payments?

* Will you be audited?

The computers here, and those scattered in 10 IRS service centers around the country, will be playing an ever more important role in tax administration in coming years, IRS Commissioner Roscoe Egger says.

''We are working on a whole system redesign,'' he told reporters at breakfast recently. By the late 1980s, when such a system might be installed, it could even perform an instant audit on all returns. In the meantime, the IRS is stepping up computer matching of information from a taxpayer's banker, broker, or employer with what a taxpayer reports.

The amount of space the IRS devotes to each individual in its master file here seems designed to promote humility. The typical taxpayer takes up just 1/4 inch of the 2,400 feet of magnetic tape on a reel of tape. It takes 1,500 reels to squeeze in some 95 million individual returns.

Coded into that quarter inch is five years worth of tax records. One reason so little space is required is that the IRS does not keep all the information you file on your return. For example, sources say, certain deductions are lumped together. IRS officials will not disclose which information is not stored in the computer.

''New tax laws seem to be passed every year. That requires we add a data element or delete an element. But we rarely ever delete categories of information, says Kenneth Edminson, executive assistant to the center's director.

Though the US tax system is still believed to be the Cadillac of world systems in terms of taxes voluntarily paid, it might more aptly be termed a Honda: The process of updating files and making final decisions on US tax returns is overseen by a computer made by the Japanese firm Hitach.

''Sometimes we think it is laughing at us,'' a worker at the center quips.

Tax returns start their journey to Martinsburg at one of the IRS regional service centers. Once a return arrives on the loading dock, it is opened and sorted by a high-speed machine that reads the bar code on the envelope. The machine shoots 30,000 returns an hour into any one of 15 different slots, depending on the type of return.

After this preliminary sorting, returns are rolled into another room to be broken down into more specialized categories. Returns with checks are next sent to a remittance processing area where funds are tabulated. In the April processing rush, checks are cashed four to five days after arriving at a center, IRS officials say.

Key information from the return -- like the number of exemptions -- is then coded and written on the face of the return. Other workers then type the code into a computer terminal. A computer at the center checks the coded data for math errors and consistency with law, for example, by making sure single taxpayers have not figured their tax using the married-person tax table.

The computer tapes holding the coded information are then flown to the Washington, D.C., airport where IRS trucks pick them up twice a day and drive them to Martinsburg.

When the taped information arrives at Martinsburg, it is first checked against written service-center records to ensure all the data has arrived safely.

''We have chased down a penny difference,'' in the written and computerized totals, sys Jack Fazenbaker, chief of the system support division. Then the data are sorted by the taxpayer's social security number and the validity of that number is checked.

After these quality checks are performed, the new data re added to a taxpayer's small section on the master file. ''All the information needed to determine your liability and refund'' is there, Mr. Fazenbaker says.

Once the master tape, which fills 1,500 reels, has been updated, the information is analyzed by computer. For example, it is checked to see if the taxpayer should get a refund, owes additional tax, or should get a letter of some kind. The return is also checked against separate information on income paid to taxpayers by banks, brokers, employers, and others.

One step in this processing involves grading the return for its potential to yield the IRS extra tax dollars if it is audited. This audit score is sent to IRS regional offices, which made the final decision on which taxpayers to audit. This year only 1.36 percent of returns are slated for audit.

Taxpayers do not live forever on tape in Martinsburg. At the beginning of each new year, the sixth and oldest year on tape is dropped from the master file. But it is printed out on microfilm and sent to the appropriate service center. Martinsburg also sends the service center a computerized update on any action it takes on a taxpayer in their area.

The 440 people who work at Matinsburg are surrounded each day by highly personal details of taxpayers' lives. but they have little chance to indulge their curiosity since all the information is coded.

''You couldn't do anything with it without an interpreter program,'' IRS official Edminson says. And no tape can be printed out without written authorization.