The money supply averaged $338.6 billion in the week ended March 15, the Federal Reserve Board reported yesterday, down from the $339.2 billion average in the week ended March 8.

The central bank also reported revised money supply data for 1977 that showed that checking accounts and currency in circulation grew 7.8 percent last year, rather than the 7.4 percent rise the agency originally calculated. THe new "benchmark" measures reflect data from banks that are not members of the Fed system as well as revisions in seasonal adjustments.

The Federal Reserve also announced yesterday formation of a committee of outside economists to review the Fed's seasonal adjustment techiques. The committee will be chaired by Geoffrey Moore, of the National Bureau of Economic Research and former Commissioner of Labor Statistics.

The money supply data reported yesterday showed that the money supply has been growing at an annual rate of 4.4 percent over the last 13 weeks, within the range of 4 to 6.5 percent targeted by the central bank. The federal funds rate, the interest banks pay on overnight loans of excess reserves and a key tool of Federal Reserve monetary policy, averages 6.77 percent in the week ended March 22, the same as in the week ended March 15.