Federal Reserve economist Claudia Sahm came up with a simple formula that would have accurately called every recession since 1970 within two to four months of when it started. And that's with no false positives.The RuleIf the average of unemployment rate over three months rises a half-percentage point or more above its low over the previous year, the economy is in a recession. In conjunction with that rule, she has also proposed policies to immediately soften the downturn without the political hurdles that usually slow stimulus efforts. For now, the so-called Sahm Rule is sending a reassuring signal: The economy may be slowing but no recession has begun.