Inside Report (6)

Warning: for internal use only!

That's the label on the ''sort-of'' projection of the state of the US economy. The so-called GNP ''flash'' publicized earlier this week showed the economy sliding backward at a 2.2 percent annual rate for the last three months of '82. But US Commerce Department officials say the flash is for internal use only; they cringe at the number's ''leak'' every quarter. The flash is based on 2/3 of a quarter's data for some components of the GNP, but only 1/3 of a quarter's worth of data for others. By contrast, the estimate of GNP that'll be released next month is based on 2/3 of a quarter for many of its components and a full quarter's worth of data for others. Why calculate the flash? The government's got to have something to go on when it's trying to plan ahead.

But the flash has a reasonably good track record: Since 1979, it has correctly projected quarterly growth or decline 73 percent of the time.