At the risk of being repetitive, here are some links that have been put up in past threads, plus some related ones:

On "understated" US unemployment numbers:

a piece by Paul Krugman who refers to

a study by Katharine Bradbury [pdf], of the Federal Reserve Bank of Boston, and

this post by Brad DeLong

On "understated" UK numbers:

U.K. Employment Paradise Is a Matter of Semantics by Bloomberg columnist Matthew Lynn, who refers to

The Diversion from Unemployment to Sickness [pdf], a study by researchers Christina Beatty and Stephen Fothergill of Sheffield Hallam University.