There are many models of common factors in successful psychotherapy process and outcome. Already in 1990, Grencavage and Norcross identified 89 common factors in a literature review, which showed the diversity of models of common factors. To be useful for purposes of psychotherapy practice and training, most models reduce the number of common factors to a handful, typically around five. Frank listed six common factors in 1971 and explained their interaction. Goldfried and Padawer listed five common strategies or principles in 1982: corrective experiences and new behaviors, feedback from the therapist to the client promoting new understanding in the client, expectation that psychotherapy will be helpful, establishment of the desired therapeutic relationship, and ongoing reality testing by the client. Grencavage and Norcross grouped common factors into five areas in 1990. Lambert formulated four groups of therapeutic factors in 1992. Joel Weinberger and Cristina Rasco listed five common factors in 2007 and reviewed the empirical support for each factor: the therapeutic relationship, expectations of treatment effectiveness, confronting or facing the problem (exposure), mastery or control experiences, and patients' attributions of successful outcome to internal or external causes.
Terence Tracy and colleagues modified the common factors of Grencavage and Norcross, and used them to develop a questionnaire which they provided to 16 board certified psychologists and 5 experienced psychotherapy researchers; then they analyzed the responses and published the results in 2003. Their multidimensional scaling analysis represented the results on a two-dimensional graph, with one dimension representing hot processing versus cool processing (roughly, closeness and emotional experience versus technical information and persuasion) and the other dimension representing therapeutic activity. Their cluster analysis represented the results as three clusters: the first related to bond (roughly, therapeutic alliance), the second related to information (roughly, the meanings communicated between therapist and client), and the third related to role (roughly, a logical structure so that clients can make sense of the therapy process).
In addition to these models that incorporate multiple common factors, a number of theorists have proposed and investigated single common factors, common principles, and common mechanisms of change, such as learning. In one example, at least three independent groups have converged on the conclusion that a wide variety of different psychotherapies can be integrated via their common ability to trigger the neurobiological mechanism of memory reconsolidation. For further examples, see § Further reading, below.
While many models of common factors have been proposed, they have not all received the same amount of empirical research. There is general consensus on the importance of a good therapeutic relationship in all forms of psychotherapy and counseling.
One frontier for future research on common factors is automated computational analysis of clinical big data.
Some common factors theorists have argued against these criticisms. They state that:
Some sources summarizing common factors theory include: Carr 2008, pp. 49–67; Imel & Wampold 2008; McAleavey & Castonguay 2015; Wampold & Imel 2015 - Carr, Alan (2008). What works with children, adolescents, and adults: a review of research on the effectiveness of psychotherapy. New York: Routledge. ISBN 9780415452915. OCLC 192042194. https://books.google.com/books?id=fOc2pSbobc8C
Chambless & Ollendick 2001 - Chambless, Dianne L; Ollendick, Thomas H (February 2001). "Empirically supported psychological interventions: controversies and evidence". Annual Review of Psychology. 52 (1): 685–716. doi:10.1146/annurev.psych.52.1.685. PMID 11148322. S2CID 9487499. https://www.researchgate.net/publication/12181680
Boswell et al. 2014, p. 118; this conclusion is also found in, for example, Butler & Strupp 1986, Arkowitz 1995, McAleavey & Castonguay 2015 - Boswell, James F; Sharpless, Brian A; Greenberg, Leslie S; Heatherington, Laurie; Huppert, Jonathan D; Barber, Jacques P; Goldfried, Marvin R; Castonguay, Louis G (2014). "Schools of psychotherapy and the beginnings of a scientific approach". In Barlow, David H (ed.). The Oxford handbook of clinical psychology. Oxford library of psychology (Updated ed.). New York: Oxford University Press. pp. 98–127. ISBN 9780199328710. OCLC 874118501. https://books.google.com/books?id=Cr0iAwAAQBAJ&pg=PA98
Carr 2008, p. 53 - Carr, Alan (2008). What works with children, adolescents, and adults: a review of research on the effectiveness of psychotherapy. New York: Routledge. ISBN 9780415452915. OCLC 192042194. https://books.google.com/books?id=fOc2pSbobc8C
Rosenzweig 1936, Rosenzweig 1940; Duncan 2002, p. 10; Lisa Wallner Samstag has argued that Saul Rosenzweig's contribution to common factors theory has often been misunderstood (Samstag 2002) - Rosenzweig, Saul (July 1936). "Some implicit common factors in diverse methods of psychotherapy". American Journal of Orthopsychiatry. 6 (3): 412–415. doi:10.1111/j.1939-0025.1936.tb05248.x. https://doi.org/10.1111%2Fj.1939-0025.1936.tb05248.x
Dollard & Miller 1950; more recently, Warren Tryon has championed learning as a common factor, e.g. Tryon & Tryon 2011, p. 152: "Therapists, and the therapeutic approaches that currently divide us, differ only with regard to what is to be learned and how it is to be acquired... This makes learning and memory basic to our science and profession and should motivate us to search for mechanisms that underlie all effective psychological interventions..." - Dollard, John; Miller, Neal E (1950). Personality and psychotherapy: an analysis in terms of learning, thinking, and culture. McGraw-Hill publications in psychology. New York: McGraw-Hill. OCLC 964374. https://archive.org/details/personalitypsych00doll
Garfield 1957; Duncan 2002, p. 14 - Garfield, Sol L (1957). Introductory clinical psychology: an overview of the functions, methods, and problems of contemporary clinical psychology. New York: Macmillan. OCLC 1458147. https://search.worldcat.org/oclc/1458147
Rogers 1957; 50 years later, in 2007, a series of 13 articles reviewed Rogers' 1957 article; some of those articles argued that Rogers' "sufficient conditions" are not common factors: "Special section: The necessary and sufficient conditions at the half century mark". Psychotherapy: Theory, Research, Practice, Training. 44 (3): 239–299. - Rogers, Carl R (April 1957). "The necessary and sufficient conditions of therapeutic personality change" (PDF). Journal of Consulting Psychology. 21 (2): 95–103. doi:10.1037/h0045357. PMID 13416422. Archived from the original (PDF) on 2009-02-05. Retrieved 2015-02-11. https://web.archive.org/web/20090205184306/http://person-centred.co.uk/pdf%20props/necessary%20and%20sufficient.pdf
Frank & Frank 1991; the legacy of Frank's work is discussed in Alarcón & Frank 2011 - Frank, Jerome D; Frank, Julia (1991) [1961]. Persuasion and healing: a comparative study of psychotherapy (3rd ed.). Baltimore: Johns Hopkins University Press. ISBN 978-0801840678. OCLC 21764135. https://search.worldcat.org/oclc/21764135
See also Frank 1971, p. 350: "A historical overview of Western psychotherapy reveals that the dominant psychotherapeutic approach of an era reflects contemporary cultural attitudes and values, and that the same techniques (e.g., abreaction) reappear under new names. Common to all psychotherapies are (a) an emotionally charged, confiding relationship; (b) a therapeutic rationale accepted by patient and therapist; (c) provision of new information by precept, example and self-discovery; (d) strengthening of the patient's expectation of help; (e) providing the patient with success experiences; and (f) facilitation of emotional arousal. Prevalent forms of disability and their treatment include drug therapy for constitutional vulnerabilities, emotional support for environmental crises, spiritual guidance for existential anxieties, and therapeutic maneuvers to correct faulty perceptual and behavioral habits learned early in life. Only the latter form requires therapists trained in specific psychotherapeutic methods." - Frank, Jerome D (July 1971). "Therapeutic factors in psychotherapy". American Journal of Psychotherapy. 25 (3): 350–361. doi:10.1176/appi.psychotherapy.1971.25.3.350. PMID 4936109. https://doi.org/10.1176%2Fappi.psychotherapy.1971.25.3.350
Luborsky, Singer & Luborsky 1975 - Luborsky, Lester; Singer, Barton; Luborsky, Lise (August 1975). "Comparative studies of psychotherapies: is it true that everyone has won and all must have prizes?". Archives of General Psychiatry. 32 (8): 995–1008. doi:10.1001/archpsyc.1975.01760260059004. PMID 239666. https://doi.org/10.1001%2Farchpsyc.1975.01760260059004
The question of whether all psychotherapies are all roughly equally effective (known as the Dodo bird verdict) and the question of whether all effective psychotherapies share common factors (known as common factors theory) are two different questions: "Though many authors view outcome equivalence as the main reason to study common factors in psychotherapy, we cheerfully disagree. Regardless of outcome, it is noncontroversial to say that psychotherapies of many origins share several features of process and content, and it follows that better understanding the patterns of these commonalities may be an important part of better understanding the effects of psychotherapies. That is, irrespective of whether some psychotherapies are equivalent to others in symptomatic outcome, understanding what part of clients' improvement is due to factors that are shared by several approaches appears to us to be a conceptually and clinically important question." (McAleavey & Castonguay 2015, p. 294) /wiki/Dodo_bird_verdict
Eysenck 1952 - Eysenck, Hans J (October 1952). "The effects of psychotherapy: an evaluation". Journal of Consulting Psychology. 16 (5): 319–324. doi:10.1037/h0063633. PMID 13000035. http://psychclassics.yorku.ca/Eysenck/psychotherapy.htm
For example: Smith & Glass 1977; Lipsey & Wilson 1993; Carr 2008 - Smith, Mary L; Glass, Gene V (September 1977). "Meta-analysis of psychotherapy outcome studies". American Psychologist. 32 (9): 752–760. doi:10.1037/0003-066X.32.9.752. PMID 921048. S2CID 43326263. https://www.researchgate.net/publication/22234005
For an overview of the work of some prominent psychotherapy researchers who have explored this question, see: Castonguay et al. 2010 - Castonguay, Louis G; Muran, J Christopher; Angus, Lynne E; Hayes, Jeffrey A; Ladany, Nicholas; Anderson, Timothy, eds. (2010). Bringing psychotherapy research to life: understanding change through the work of leading clinical researchers. Washington, DC: American Psychological Association. ISBN 9781433807749. OCLC 463855600. https://search.worldcat.org/oclc/463855600
Goldfried 1982 - Goldfried, Marvin R, ed. (1982). Converging themes in psychotherapy: trends in psychodynamic, humanistic, and behavioral practice. New York: Springer Pub. Co. ISBN 978-0826136213. OCLC 8169372. https://archive.org/details/convergingthemes0000unse
Goldfried 1982, pp. 3–49 - Goldfried, Marvin R, ed. (1982). Converging themes in psychotherapy: trends in psychodynamic, humanistic, and behavioral practice. New York: Springer Pub. Co. ISBN 978-0826136213. OCLC 8169372. https://archive.org/details/convergingthemes0000unse
McAleavey & Castonguay 2015, p. 298 - McAleavey, Andrew A; Castonguay, Louis G (2015). "The Process of Change in Psychotherapy: Common and Unique Factors". In Gelo, Omar CG; Pritz, Alfred; Rieken, Bernd (eds.). Psychotherapy Research. New York: Springer. pp. 293–310. doi:10.1007/978-3-7091-1382-0_15. ISBN 9783709113813. OCLC 900722269. https://books.google.com/books?id=JYTwBQAAQBAJ&pg=PA293
Prochaska, DiClemente & Norcross 1992, p. 1107 - Prochaska, James O; DiClemente, Carlo C; Norcross, John C (September 1992). "In search of how people change: applications to addictive behaviors". American Psychologist. 47 (9): 1102–1114. doi:10.1037/0003-066X.47.9.1102. PMID 1329589. https://www.researchgate.net/publication/201382100
Prochaska, DiClemente & Norcross 1992, p. 1107 - Prochaska, James O; DiClemente, Carlo C; Norcross, John C (September 1992). "In search of how people change: applications to addictive behaviors". American Psychologist. 47 (9): 1102–1114. doi:10.1037/0003-066X.47.9.1102. PMID 1329589. https://www.researchgate.net/publication/201382100
Orlinsky & Howard 1986; for a more recent summary see: Orlinsky 2017 - Orlinsky, David E; Howard, Kenneth I (1986). "Process and outcome in psychotherapy". In Garfield, Sol L; Bergin, Allen E (eds.). Handbook of psychotherapy and behavior change (3rd ed.). New York: Wiley. pp. 311–381. ISBN 978-0471799955. OCLC 13425136. https://archive.org/details/handbookofpsycho00garf
Grencavage & Norcross 1990 - Grencavage, Lisa M; Norcross, John C (October 1990). "Where are the commonalities among the therapeutic common factors?". Professional Psychology: Research and Practice. 21 (5): 372–378. doi:10.1037/0735-7028.21.5.372. https://www.researchgate.net/publication/232519607
Beutler & Clarkin 1990 - Beutler, Larry E; Clarkin, John F (1990). Systematic treatment selection: toward targeted therapeutic interventions. Brunner/Mazel integrative psychotherapy series. Vol. 3. New York: Brunner/Mazel. ISBN 978-0876305768. OCLC 20826889. https://books.google.com/books?id=uovFBQAAQBAJ
Beutler, Moleiro & Talebi 2002 - Beutler, Larry E; Moleiro, Carla; Talebi, Hani (October 2002). "How practitioners can systematically use empirical evidence in treatment selection". Journal of Clinical Psychology. 58 (10): 1199–1212. doi:10.1002/jclp.10106. PMID 12357437. https://doi.org/10.1002%2Fjclp.10106
Lambert 1992 - Lambert, Michael J (1992). "Psychotherapy outcome research: implications for integrative and eclectic therapists". In Norcross, John C; Goldfried, Marvin R (eds.). Handbook of psychotherapy integration (1st ed.). New York: Basic Books. pp. 94–129. ISBN 978-0465028795. OCLC 25547822. https://archive.org/details/isbn_9780465028795/page/94
Duncan, Hubble & Miller 2010 - Duncan, Barry L; Hubble, Mark A; Miller, Scott D, eds. (2010) [1999]. The heart & soul of change: delivering what works in therapy (2nd ed.). Washington, DC: American Psychological Association. ISBN 9781433807091. OCLC 370605648. https://search.worldcat.org/oclc/370605648
Wampold & Imel 2015, ch. 1 - Wampold, Bruce E; Imel, Zac E (2015) [2001]. The great psychotherapy debate: the evidence for what makes psychotherapy work (2nd ed.). New York: Routledge. ISBN 9780805857092. OCLC 227918397. https://books.google.com/books?id=ljZyBgAAQBAJ
Chambless & Ollendick 2001 - Chambless, Dianne L; Ollendick, Thomas H (February 2001). "Empirically supported psychological interventions: controversies and evidence". Annual Review of Psychology. 52 (1): 685–716. doi:10.1146/annurev.psych.52.1.685. PMID 11148322. S2CID 9487499. https://www.researchgate.net/publication/12181680
Wampold & Imel 2015 - Wampold, Bruce E; Imel, Zac E (2015) [2001]. The great psychotherapy debate: the evidence for what makes psychotherapy work (2nd ed.). New York: Routledge. ISBN 9780805857092. OCLC 227918397. https://books.google.com/books?id=ljZyBgAAQBAJ
For example: Sprenkle & Blow 2004; Davis & Piercy 2007; Blow et al. 2009; Sprenkle, Davis & Lebow 2009; Lebow 2014; Karam et al. 2015 - Sprenkle, Douglas H; Blow, Adrian J (April 2004). "Common factors and our sacred models". Journal of Marital & Family Therapy. 30 (2): 113–129. doi:10.1111/j.1752-0606.2004.tb01228.x. PMID 15114942. https://doi.org/10.1111%2Fj.1752-0606.2004.tb01228.x
"Special section: Common factors". Psychotherapy: Theory, Research, Practice, Training. 51 (4): 476–524. http://psycnet.apa.org/journals/pst/51/4/
"Special section: Common factors". Psychotherapy: Theory, Research, Practice, Training. 51 (4): 476–524. http://psycnet.apa.org/journals/pst/51/4/
Hofmann & Barlow 2014, p. 511; see also Hofmann 2019 - Hofmann, Stefan G; Barlow, David H (December 2014). "Evidence-based psychological interventions and the common factors approach: the beginnings of a rapprochement?". Psychotherapy: Theory, Research, Practice, Training. 51 (4): 510–513. doi:10.1037/a0037045. PMID 25111379. https://www.researchgate.net/publication/264628212
For example: Tschacher, Junghan & Pfammatter 2014 - Tschacher, Wolfgang; Junghan, Ulrich Martin; Pfammatter, Mario (January 2014). "Towards a taxonomy of common factors in psychotherapy—results of an expert survey". Clinical Psychology & Psychotherapy. 21 (1): 82–96. doi:10.1002/cpp.1822. PMID 23129553. https://doi.org/10.1002%2Fcpp.1822
Grencavage & Norcross 1990 - Grencavage, Lisa M; Norcross, John C (October 1990). "Where are the commonalities among the therapeutic common factors?". Professional Psychology: Research and Practice. 21 (5): 372–378. doi:10.1037/0735-7028.21.5.372. https://www.researchgate.net/publication/232519607
See also Frank 1971, p. 350: "A historical overview of Western psychotherapy reveals that the dominant psychotherapeutic approach of an era reflects contemporary cultural attitudes and values, and that the same techniques (e.g., abreaction) reappear under new names. Common to all psychotherapies are (a) an emotionally charged, confiding relationship; (b) a therapeutic rationale accepted by patient and therapist; (c) provision of new information by precept, example and self-discovery; (d) strengthening of the patient's expectation of help; (e) providing the patient with success experiences; and (f) facilitation of emotional arousal. Prevalent forms of disability and their treatment include drug therapy for constitutional vulnerabilities, emotional support for environmental crises, spiritual guidance for existential anxieties, and therapeutic maneuvers to correct faulty perceptual and behavioral habits learned early in life. Only the latter form requires therapists trained in specific psychotherapeutic methods." - Frank, Jerome D (July 1971). "Therapeutic factors in psychotherapy". American Journal of Psychotherapy. 25 (3): 350–361. doi:10.1176/appi.psychotherapy.1971.25.3.350. PMID 4936109. https://doi.org/10.1176%2Fappi.psychotherapy.1971.25.3.350
Goldfried 1982, pp. 3–49 - Goldfried, Marvin R, ed. (1982). Converging themes in psychotherapy: trends in psychodynamic, humanistic, and behavioral practice. New York: Springer Pub. Co. ISBN 978-0826136213. OCLC 8169372. https://archive.org/details/convergingthemes0000unse
Grencavage & Norcross 1990 - Grencavage, Lisa M; Norcross, John C (October 1990). "Where are the commonalities among the therapeutic common factors?". Professional Psychology: Research and Practice. 21 (5): 372–378. doi:10.1037/0735-7028.21.5.372. https://www.researchgate.net/publication/232519607
Lambert 1992 - Lambert, Michael J (1992). "Psychotherapy outcome research: implications for integrative and eclectic therapists". In Norcross, John C; Goldfried, Marvin R (eds.). Handbook of psychotherapy integration (1st ed.). New York: Basic Books. pp. 94–129. ISBN 978-0465028795. OCLC 25547822. https://archive.org/details/isbn_9780465028795/page/94
Weinberger & Rasco 2007 - Weinberger, Joel L; Rasco, Cristina (2007). "Empirically supported common factors". In Hofmann, Stefan G; Weinberger, Joel L (eds.). The art and science of psychotherapy. New York: Routledge. pp. 103–129. ISBN 978-0415952156. OCLC 71241861. https://books.google.com/books?id=_YqVBhAuLDUC&pg=PA101
Tracey et al. 2003, pp. 406–410 - Tracey, Terence GJ; Lichtenberg, James W; Goodyear, Rodney K; Claiborn, Charles D; Wampold, Bruce E (December 2003). "Concept mapping of therapeutic common factors". Psychotherapy Research. 13 (4): 401–413. doi:10.1093/ptr/kpg041. PMID 21827252. https://www.researchgate.net/publication/245536619
Tracey et al. 2003, pp. 406–410 - Tracey, Terence GJ; Lichtenberg, James W; Goodyear, Rodney K; Claiborn, Charles D; Wampold, Bruce E (December 2003). "Concept mapping of therapeutic common factors". Psychotherapy Research. 13 (4): 401–413. doi:10.1093/ptr/kpg041. PMID 21827252. https://www.researchgate.net/publication/245536619
Ecker, Ticic & Hulley 2012; Welling 2012; Lane et al. 2015; for a more hesitant view of the role of memory reconsolidation in psychotherapy see Alberini 2015 and the objections in some of the invited comments in Lane et al. 2015 - Ecker, Bruce; Ticic, Robin; Hulley, Laurel (2012). Unlocking the emotional brain: eliminating symptoms at their roots using memory reconsolidation. New York: Routledge. ISBN 9780415897167. OCLC 772112300. https://books.google.com/books?id=fQpd13K0gHUC
Laska, Gurman & Wampold 2014, p. 472 (Table 1) - Laska, Kevin M; Gurman, Alan S; Wampold, Bruce E (December 2014). "Expanding the lens of evidence-based practice in psychotherapy: a common factors perspective". Psychotherapy: Theory, Research, Practice, Training. 51 (4): 467–481. doi:10.1037/a0034332. PMID 24377408. https://www.researchgate.net/publication/259497860
McAleavey & Castonguay 2015, pp. 301–302 - McAleavey, Andrew A; Castonguay, Louis G (2015). "The Process of Change in Psychotherapy: Common and Unique Factors". In Gelo, Omar CG; Pritz, Alfred; Rieken, Bernd (eds.). Psychotherapy Research. New York: Springer. pp. 293–310. doi:10.1007/978-3-7091-1382-0_15. ISBN 9783709113813. OCLC 900722269. https://books.google.com/books?id=JYTwBQAAQBAJ&pg=PA293
Imel & Wampold 2008, p. 255 - Imel, Zac E; Wampold, Bruce E (2008). "The importance of treatment and the science of common factors in psychotherapy". In Brown, Steven D; Lent, Robert W (eds.). Handbook of counseling psychology (4th ed.). Hoboken, NJ: John Wiley & Sons. pp. 249–262. ISBN 9780470096222. OCLC 145942481. https://books.google.com/books?id=oF_CwsBO8kQC&pg=PA249
Laska, Gurman & Wampold 2014, p. 472 (Table 1) - Laska, Kevin M; Gurman, Alan S; Wampold, Bruce E (December 2014). "Expanding the lens of evidence-based practice in psychotherapy: a common factors perspective". Psychotherapy: Theory, Research, Practice, Training. 51 (4): 467–481. doi:10.1037/a0034332. PMID 24377408. https://www.researchgate.net/publication/259497860
Kazdin 2005; Kazdin 2009; similarly, see also Rosen & Davison 2003 - Kazdin, Alan E (June 2005). "Treatment outcomes, common factors, and continued neglect of mechanisms of change". Clinical Psychology: Science and Practice. 12 (2): 184–188. doi:10.1093/clipsy.bpi023. https://doi.org/10.1093%2Fclipsy.bpi023
Imel & Wampold 2008, p. 261 - Imel, Zac E; Wampold, Bruce E (2008). "The importance of treatment and the science of common factors in psychotherapy". In Brown, Steven D; Lent, Robert W (eds.). Handbook of counseling psychology (4th ed.). Hoboken, NJ: John Wiley & Sons. pp. 249–262. ISBN 9780470096222. OCLC 145942481. https://books.google.com/books?id=oF_CwsBO8kQC&pg=PA249
Dollard & Miller 1950; more recently, Warren Tryon has championed learning as a common factor, e.g. Tryon & Tryon 2011, p. 152: "Therapists, and the therapeutic approaches that currently divide us, differ only with regard to what is to be learned and how it is to be acquired... This makes learning and memory basic to our science and profession and should motivate us to search for mechanisms that underlie all effective psychological interventions..." - Dollard, John; Miller, Neal E (1950). Personality and psychotherapy: an analysis in terms of learning, thinking, and culture. McGraw-Hill publications in psychology. New York: McGraw-Hill. OCLC 964374. https://archive.org/details/personalitypsych00doll
Frank & Frank 1991; the legacy of Frank's work is discussed in Alarcón & Frank 2011 - Frank, Jerome D; Frank, Julia (1991) [1961]. Persuasion and healing: a comparative study of psychotherapy (3rd ed.). Baltimore: Johns Hopkins University Press. ISBN 978-0801840678. OCLC 21764135. https://search.worldcat.org/oclc/21764135
For example: Imel, Steyvers & Atkins 2015; Chen & Wojcik 2016 - Imel, Zac E; Steyvers, Mark; Atkins, David C (March 2015). "Computational psychotherapy research: scaling up the evaluation of patient–provider interactions". Psychotherapy: Theory, Research, Practice, Training. 52 (1): 19–30. doi:10.1037/a0036841. PMC 4245387. PMID 24866972. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4245387
Imel & Wampold 2008, pp. 256–258 - Imel, Zac E; Wampold, Bruce E (2008). "The importance of treatment and the science of common factors in psychotherapy". In Brown, Steven D; Lent, Robert W (eds.). Handbook of counseling psychology (4th ed.). Hoboken, NJ: John Wiley & Sons. pp. 249–262. ISBN 9780470096222. OCLC 145942481. https://books.google.com/books?id=oF_CwsBO8kQC&pg=PA249
Imel & Wampold 2008, pp. 258–260 - Imel, Zac E; Wampold, Bruce E (2008). "The importance of treatment and the science of common factors in psychotherapy". In Brown, Steven D; Lent, Robert W (eds.). Handbook of counseling psychology (4th ed.). Hoboken, NJ: John Wiley & Sons. pp. 249–262. ISBN 9780470096222. OCLC 145942481. https://books.google.com/books?id=oF_CwsBO8kQC&pg=PA249