Researchers examined 100 people suffering from herniated disks in their backs. Five of them were found to have a defect in a particular gene. The researchers also examined 100 people who had no problems with the disks in their backs; none had the genetic defect. They concluded that the genetic defect increases the likelihood of herniated disks.
The statement "When the researchers examined the families of the 5 subjects who had the defective gene, they found that 30 family members also had the defective gene, and each of them suffered from herniated disks" most strengthens the researchers' reasoning.