So the participation rate rose by 0.1 percentage points between August 2012 and September 2012. The labour force grew (418 thousand workers were added) but employment added 873 thousand, which meant that unemployment fell by 455 thousand, and consequently the unemployment rate (which is the percentage of workers in the labour force that are unemployed) fell to 7.8 per cent. The labour force growth outstripped the underlying growth in the working age population because more of the working age population chose to participate in September. What was the impact of the participation rate increase in September 2012? Note the final column in the Table above. It calculates what the labour force would have been, given the underlying population growth, had the participation remained at the lower August figure. So it would have only grown by 174 thousand (as a result of underlying growth in the population) instead of 418 thousand. The following Table shows you what that means. So the increasing participation rate added 244 thousand more workers to the labour force beyond the underlying population growth, which added 174 thousand. Had the participation rate not increased, the unemployment rate would have fallen to 7.65 per cent rather than 7.8 per cent. This all rests on employment growing by 873 thousand. How sure can we be of that? The BLS provide a lot of information about the household data estimates in this document -- Household data, reliability of estimates . You will see that for total employment their standard error