Each person, depending on many factors, such as physical conditioning, amount of body fat, and thickness of clothing, has vulnerable or invulnerable areas. Do not assume that because a particular target strike is effective on one person that the same strike will be effective on another person. A street fighter or a boxer is used to receiving head, rib, and abdomen strikes. Also, not all people react to pain in the same way. For some, pain is pleasure and will only serve to encourage brutally. When in doubt, go for the most crippling strike possible and let a court of law prove that, as a victim, you were NOT in fear for your life.
When defending yourself remember to SING. Attack the Solar Plexus, Instep, Nose, and Groin.