Beware of Selection Bias
Lets travel back in time to World War II. You are an analyst and you discovered that your aircrafts has low survival rate. Only 10% of the aircrafts made it back from missions. You examined the aircrafts and discovered that the enemy has good marksmen and all the shots landed in the cockpit. 90% of the bullet holes are on the left cockpit. 10% are on the right cockpit. You can only armor plate one side of the aircraft due to weight constrain. Which side would you armor plate?