real world evidence unions cause unemployment

Where can I find actual hard evidence and not theory that unions cause unemployment?

the 70s.

First you’d have to find a territory without unions and count the unemployed in it. Then you’d have to go back in time and organize unions over the same territory and same period of time while keeping everything else the same, then count the unemployed for that scenario. If the latter number is larger than the former you would then have evidence that unions cause unemployment.

Detroit