Across two general population (total n=1,878) and two cancer (total n=3,140) samples, we evaluated the dimensionality of self-reported fatigue as measured by the Functional Assessment of Chronic Illness Therapy--Fatigue (FACIT-F) instrument. After evaluating dimensionality of the FACIT-F, we compared the conceptually distinct fatigue experience versus fatigue impact scores in each sample. Confirmatory factor analysis of the 13-item scale showed very good fit to a single dimension ("unidimensional") model for each sample (comparative fit index range=0.92-0.97). Using a bifactor model to compare the loading of each item with the general fatigue factor versus the identified sub-domain (experience or impact), we found the item-general loading to be higher than that of the item-sub-domain factor in 52 of 52 comparisons (13 items; four samples). When scored separately, experience and impact scores were correlated highly (range=0.80-0.88), yet their difference relative to one another was significant (p<0.001). Consistently across samples, experience scores were systematically higher (more endorsement) than impact scores, by a margin of 0.21-0.46 SD units. This suggests that the fatigue experience and the impact of fatigue upon function are reported along a single dimensional continuum, but that experience is more likely than impact upon function to be endorsed at lower levels of fatigue. Fatigue as an outcome or trial endpoint can be expressed as a single number, and the experience of the symptom is more likely to be endorsed at mild levels of fatigue, presumably before the symptom exerts an adverse impact upon function.