confusion of the inverse occurs when we incorrectly believe ▼ upper p left parenthesis b|a right parenthesis equals startfraction upper p left parenthesis upper a and upper b right parenthesis over upper p left parenthesis upper a right parenthesis endfraction . upper p left parenthesis b|a right parenthesis equals upper p left parenthesis a|b right parenthesis . upper p left parenthesis upper a right parenthesis plus upper p left parenthesis upper a overbar right parenthesis equals 1. upper p left parenthesis at least one right parenthesis equals 1 minus upper p left parenthesis none right parenthesis .