The Census Bureau said yesterday that part of the sharp drop in the poverty rate in 1984 that it reported last month was the result of a new method of calculating interest income.

It yields a higher estimate of income received from interest. The bureau, in a clarification of its Aug. 28 report on poverty and income report, said that if the same method been used in 1983, Americans' income for that year would have been reported as higher and the poverty rate slightly lower. Consequently, the decline in poverty in 1984 as compared with 1983, while still sharp, would not have been quite as steep as it was estimated in the bureau's recent report.

The bureau said:

*Using the new method, it found that in 1983, 15.2 percent of the total U.S. population was below the poverty line, compared with 14.4 percent in 1984, a decline of 0.8 percentage points. Originally, the bureau had reported that the 1983 figure was 15.3 percent.

*For persons 65 and over, who receive a much larger portion of their income from interest than do other groups, the bureau reported yesterday that 13.8 percent were below the poverty line in 1983, compared with 12.4 percent in 1984, a drop of 1.4 percentage points. On Aug. 28, the bureau had said the 1983 figure was 14.2 percent.

*Median family income was initially reported as rising from $25,594 in constant dollars in 1983 to $26,433 in 1984, a jump of 3.3 percent. But applying the new methodology, the figure for 1983 should have been $25,724, for an increase of 2.8 percent.