From George Washington to 1864, assassination attempts were made on 1 in 4 presidents and 1 in 16 presidents were killed. Since 1865, assassination attempts were made on 1 in 4 presidents and 1 in 7 have been killed. In the last 30 years, three attacks have been made on the presidents - none of these were successful. Overall, since 1864 the frequency of assassination attempts has remained the same while the success rate has nearly doubled.
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Source:
Security Degree Hub [4]