A study hаs fоund pоsitive benefits fоr individuаls who pаrticipated in a 10-week exercise program on a range of key outcome variables. The study sample consisted of younger adults between the ages of 25-35, primarily white, upper socio-economic status, and residing in an urban setting. The researchers are interested in generalizing the positive results to all adults across the lifespan and from all different socio-demographic backgrounds. Which threat to external validity should they consider?