This post takes an historical look at the difference between the unemployment rate as calculated using the civilian labor force at the peak of the business cycle and the headline unemployment rate.  If the economy was in a recession (NBER dates), then I calculated the unemployment rate as:

1 - (employment / civilian labor force at the last peak)

As a general rule going back to 1948, the difference between the above calculation and the headline unemployment rate tended to be negative, although there were a couple of short-lived minor recessions where this pattern did not occur.  In the figure below the green rectangles indicate NBER recessions (USRECM) and the orange/brown line (diffurate) indicates the difference between the calculation shown above and the headline unemployment rate.  Obviously the brown line shows no difference (beyond rounding, hence the squiggles) when the economy is not in recession.  

It's obvious from this figure that something is radically different in the relationship between the unemployment rate and the civilian labor force.  Even when the economy suffered large employment shocks, as happened during the 1970s, the pattern showed a negative difference between the unemployment rate calculated using the cyclical peak civilian labor force and the headline unemployment rate.  This is the only major recession in which that pattern does not hold.  Why?