Strikes data for #3.21

Year  Number of strikes

1951  4737
1952  5117
1953  5091
1954  3468
1955  4320
1956  3825
1957  3673
1958  3694
1959  3708
1960  3333
1961  3367
1962  3614
1963  3362
1964  3655
1965  3963
1966  4405
1967  4595
1968  5045
1969  5700 
1970  5716
1971  5138
1972  5010
1973  5353
1974  6074
1975  5031
1976  5648
1977  5506
1978  4230
1979  4827
1980  3885