Doctor: Medical researchers recently examined a large group of individuals who said that they had never experienced serious back pain  Half of the members of the group turned out to have bulging or slipped disks in their spines, conditions often blamed for serious back pain
Since these individuals with bulging or slipped disks evidently felt no pain from them, these conditions could not lead to serious back pain in people who do experience such pain
A characteristic found in half of a given sample of the population might not occur in half of the entire population.