From the BLS recent information on Major Work Stoppages in 2013
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You can see that the frequency of strikes was huge back in the 50's and 60's compared to nowadays. And these are only the large strikes of 1000 or more people that lasted at least a full work shift or longer. The # of stoppages peaked at 470 in 1952 and hit a low of just 5 in 2009. In 2013 there were 13 strikes. Every day or so there was a large strike back in the 50's but now its only about once a month.