WASHINGTON (AP) _ Like members of Congress and high-ranking federal executive officials, federal judges are required to file annual statements of what they bought, sold, earned, owned or owed during the year.

The Associated Press transferred the data for a complete year of judge's forms, 1987, onto computers for analysis. More than 20,000 pieces of data were included.

The reporting rules contained in the Ethics in Government Act of 1978 blur the actual value of the investments and income being disclosed. Most entries are reported within wide numerical ranges rather than in exact dollar amounts.

The forms understate assets by exempting some from disclosure, including:

-Any home, which is the largest asset for most Americans.

-Any bank account under $5,000.

-Any asset under $1,000.

-Any car or personal property.

-Any property not held for investment purposes or to produce income.

In addition, property can be valued at cost, which is often decades out of date.

Liabilities exempt from reporting include home mortgages, debts to relatives and revolving charge accounts with balances below $10,000.

For example, a rental property worth $90,000 and generating rental income of $10,144 a year would be listed as being worth ''N'' and the income would be ''L.'' N stands for $50,001 to $100,000 and L is $5,001 to $15,000.

As the number of items handled this way increases, the overall precision of the forms decreases.

It is least precise at the high end of the ranges, which stop at ''over $250,000'' for assets and ''over $100,000'' for income. A $10 million property would be valued as ''P,'' which simply signifies more than $250,000. If it brought in income of $1 million annually, that would be listed as ''H,'' more than $100,000.

Net worth cannot be calculated from these forms. Instead, the forms provide data to calculate a net value for an official's investment portfolio by subtracting reported liabilities from reported assets.

The process can be confusing, even to judges - a handful of whom attached complaining notes to their forms. In some cases, where the form was filled out wrong but the intent was clear, minor interpretation was used by the AP in summarizing a judge's finances. In cases where more than one interpretation was possible, the finances were recorded exactly as shown on the form.

The reports examined by the AP were filed in 1988 and cover the judges' finances for 1987. That is the most recent complete set. Reports are required of all judges who work at least 60 days during a calendar year; between 17 percent and 20 percent of senior judges do not work that much.

Some of the judges filing then have since retired, some are living elsewhere, and a few have died. Other judges have since gone onto the bench, but their finances are not part of this study because they were not required to file for that year.