Using a series of molecular methods, the scientists measured each individual's rates of aging. One of the techniques used was an epigenetic clock, designed by Prof. Horvath in 2013.
Prof. Horvath's epigenetic clock works by tracking methylation. Methylation is a process where methyl groups are added to DNA; this generally leads to a reduction in gene transcription, altering a person's phenotype without altering their genotype.
Methylation is natural and steadily occurs over time; the team found that by comparing an individual's chronological age with their blood's biological age, they could predict life expectancy.