1. S. Rotmann, L. Mundaca, R. Castaño-Rosa, K. O’Sullivan, A. Ambrose, R. Marchand, M. Chester, B. Karlin, K. Ashby, D. Butler, J. Chambers, Hard-to- Reach Energy Users: A Literature Review, SEA - Sustainable Energy Advice Ltd., Wellington, 2020.
2. R. Lindenberg, Hard to reach: client or casework agency? Soc. Work 3 (1958) 23–29.
3. H. Beder, Reaching the hard-to-reach adult through effective marketing, in: New Directions for Continuing Education 8, 1980, pp. 11–26.
4. Casework with hard-to-reach families;Tinker;Am. J. Orthopsychiatry,1959
5. HTR Characterisation;Ashby,2020