Cognitive behavioral therapy (CBT) is an evidence-based psychotherapy that has gained significant acceptance and influence in the treatment of depressive and anxiety disorders and is recommended as a first-line treatment for both of these [1,2]. It has also been shown to be as effective as medications in the treatment of a number of psychiatric illnesses [3-6]. Homework is an important component of CBT; in the context of CBT, homework can be defined as “specific, structured, therapeutic activities that are routinely discussed in session, to be completed between sessions” [7]. Completion of homework assignments was emphasized in the conception of CBT by its creator, Aaron Beck [8]. Many types of homework are prescribed by CBT practitioners, including symptom logs, self-reflective journals, and specific structured activities like exposure and response prevention for obsessions and compulsions. These can be divided into the following 3 main categories: (1) psychoeducational homework, (2) self-assessment homework, and (3) modality-specific homework. Psychoeducation is an important component in the early stage of therapy. Reading materials are usually provided to educate the client on the symptomatology of the diagnosed illness, its etiology, as well as other treatment-relevant information. Self-assessment strategies, including monitoring one’s mood using thought records, teach the patients to recognize the interconnection between one’s feelings, thoughts, and behaviors [8]. For example, depressed patients may be asked to identify thinking errors in daily life and document the negative influences these maladaptive thinking patterns can produce on their behaviors. Various psychiatric disorders may require different types of modality-specific homework. For example, exposure to images of spiders is a treatment method specific to arachnophobia, an example of a “specific phobia” in the Diagnostic and Statistical Manual of Mental Disorders, 5th Edition (DSM-5) [9]. Homework is strategically created by the therapist to correct and lessen the patient’s psychopathology. The purpose of these exercises is to allow the patients to practice and reinforce the skills learned in therapy sessions in real life.

Homework non-compliance is one of the top cited reasons for therapy failure in CBT [10] and has remained a persistent problem in the clinical practice. Surveys of practitioners have suggested rates of non-adherence in adult clients of approximately 20% to 50% [10,11] while adherence rates in adolescents have been reported to be approximately 50% [12]. Many barriers to homework compliance have been identified in the literature; to facilitate discussions, they can be divided into internal and external factors. Internal factors originate from a client’s own psychological environment while external ones are created by external influences. Internal factors that have been identified include lack of motivation to change the situation when experiencing negative feelings, the inability to identify automatic thoughts, disregard for the importance or relevance of the homework, and the need to see immediate results [12-14]. Various external factors have also been identified, including the effort associated with pen-and-paper homework formats, the inconvenience of completing homework because of the amount of time consumed, not understanding of the purpose of the homework, lack of instruction, and failure to anticipate potential difficulties in completing the homework [14-16]. There is strong evidence suggesting that homework compliance is integral to the efficacy of CBT in a variety of psychiatric illnesses. In the treatment of depression with CBT, homework compliance has been correlated with significant clinical improvement and shown to predict decreases in both subjective and objective measures of depressive symptoms [17-23]. Similarly, homework compliance is correlated with short-term and long-term improvement of symptoms in anxiety disorders, including generalized anxiety disorder (GAD), social anxiety disorder (SAD), hoarding, panic disorder, and post-traumatic stress disorder (PTSD) [17,24-32]. Fewer studies have been done on homework compliance in other psychiatric conditions, but better homework compliance has been correlated with significant reductions in pathological behaviors in psychotic disorders [33,34], cocaine dependence [35,36], and smoking [37]. Two meta-analyses further support the notion that greater homework adherence is associated with better treatment outcomes in depression, anxiety-related disorders, and substance use [38,39].

Despite its demonstrated efficacy, access to CBT (as well as other forms of psychotherapy) remains difficult due to the limited number of practicing psychotherapists and the cost of therapy sessions [40]. With the rise of mass-market mobile communication devices such as the iPhone or other kinds of mobile devices with app capabilities (smartphones), new solutions are being sought that will use these devices to provide therapy to patients in a more cost-effective manner. Mobile phones with app capabilities are portable devices that combine features of a cellphone and a hand-held computer with the ability to wirelessly access the Internet. Over time, ownership of mobile phones in North America has grown [41,42] and progressively lower prices have further reduced barriers to their use and ownership [43,44]. As more and more people acquire mobile phones, the acceptance of and the demand for mobile health solutions have been on the rise [45]. Boschen (2008), in a review predating the popularization of the modern mobile phone, identified the unique features of the mobile telephone that made it a potentially suitable vehicle for adjunctive therapeutic applications: portability, acceptability, low initial cost, low maintenance cost, social penetration and ubiquity, “always on,” “always connected,” programmability, audio and video output, keypad and audio input, user-friendliness, and ease of use [46]. Over the last decade, modern mobile phones have supplanted the previous generation of mobile telephones; progressive increases in their computing power, ongoing advances in the software that they run and interact with (eg, JAVA, HTML5, etc.), common feature sets across different operating systems such as Google Inc.'s Android or Apple Inc.'s iOS, and adoption of common hardware elements across manufacturers (eg, touch screens, high-resolution cameras, etc) have enabled the development of platform-independent apps for mobile phones, or at least apps on different platforms with comparable functionality (eg, apps written for Apple's HealthKit or the apps written for Microsoft's HealthVault).

The popularization of the smartphone presents a unique opportunity to enhance CBT homework compliance using adjunctive therapeutic applications such that well-designed mobile software may be able to diminish barriers to CBT [40] by making CBT therapists' work more cost-effective. However, there are no guidelines and no existing research that directly address the design of mobile phone apps for this purpose. Given this gap in the literature, we searched MEDLINE (1946 to April 2015) and PsycINFO (1806 to April 2015) for all articles related to “cognitive behavioral therapy”, “homework”, “mobile applications” and “treatment compliance or adherence”, and reviewed articles related to (1) mobile technologies that address homework completion, (2) essential features of therapy, or (3) barriers to homework completion in CBT. In this article, we propose a collection of essential features for mobile phone-based apps that will optimally support homework compliance in CBT.

In order to be effective for patients and acceptable to therapists, an optimal mobile phone app to support CBT homework compliance should conform to the CBT model of homework while addressing barriers to homework compliance. Tompkins (2002) provides a comprehensive guideline on the appropriate ways to provide CBT homework such that homework should be meaningful, relevant to the central goals of therapy, salient to focus of the session, agreeable to both therapist and client, appropriate to sociocultural context, practiced in session to improve skill, doable, begin small, have a clear rationale, include written instructions, and include a backup plan with homework obstacles [47]. In addition, the therapist providing the homework needs to be curious, collaborative, reinforce all pro-homework behavior and successful homework completion, and emphasize completion over outcome [47]. By combining Tompkins' guidelines with the need to reduce barriers to homework compliance (as described above), we obtained the following list of 6 essential features that should be incorporated into mobile apps to maximize homework compliance: (1) congruency to therapy, (2) fostering learning, (3) guiding therapy, (4) building connections, (5) emphasizing completion, and (6) population specificity.

There are several additional issues specific to mobile apps that should be carefully considered when developing mobile apps for homework compliance. Because of screen sizes, input modes, the nature of electronic media, etc, standard CBT homework may need to be translated or modified to convert it into a format optimal for delivery via a mobile phone [47]. The inclusion of text messaging features remains controversial, in part because of concerns about client-therapist boundary issues outside the therapy sessions [90]. One potential solution is to use automated text messaging services to replace direct communication between the therapist and the client so the therapist can't be bombarded by abusive messages [52,61,91,92]. Privacy and security issues are also real concerns for the users of technology [93], although no privacy breaches related to text messaging or data security have been reported in studies on mobile apps so far [88,94-98]. Designers of mobile apps should ensure that any sensitive health-related or personal data is stored securely, whether on the mobile device or on a server.

Finally, while this paper focused on “essential” features of apps, this should not be misunderstood as an attempt to itemize all elements necessary for designing a successful piece of software. Good software design depends on many important elements that are beyond the scope of this paper, such as a well-designed user interface [99] that is cognitively efficient relative to its intended purpose [100] and which makes effective use of underlying hardware.

The popularization and proliferation of the mobile phone presents a distinct opportunity to enhance the success rate of CBT by addressing the pervasive issue of poor homework compliance. A variety of barriers exist in traditional, paper-based CBT homework that can significantly hamper clients’ motivation to complete homework as directed. The 6 essential features identified in this paper can each potentially enhance homework compliance. Therapy congruency focuses the features of the app on the central goal of therapy and fostering learning eases engagement in therapy by reducing barriers. Apps should help the therapist guide the client through therapy and not hinder the therapeutic process or interfere with patient’s building connections with others. It is crucial that homework completion be emphasized by the app, not just homework attempting. Population-specific issues should also be considered depending on the characteristics of targeted users.

As an example of how this applies in practice, “Mental Health Telemetry-Anxiety Disorders” (MHT-ANX) is a new mobile app developed by the Centre for Mobile Computing in Mental Health at Sunnybrook Health Sciences Centre in Toronto that helps patients monitor their anxiety symptoms using longitudinal self-report. The symptom log is therapy congruent to the practice of CBT since it promotes patients' awareness of their anxiety symptoms and the symptoms’ intensity. The simplicity of the app makes it easy for patients to learn to use, consistent with the need for fostering learning and increasing compliance. The MHT-ANX app was designed to share patient data with their clinicians, helping clinicians guide patients through therapy and more readily engage in discussion about symptom records, thus potentially enhancing the therapeutic relationship. Homework completion is emphasized both by automated text message reminders that the system sends and by questions presented by MHT-ANX that focus on how homework was done. While there are few population-specific design issues obvious at first glance in MHT-ANX, the focus groups conducted as part of our design process highlighted that our target group preferred greater privacy in our app rather than ease of sharing results via social media, and prioritized ease-of-use. While not yet formally assessed, reports from staff and early users suggest that MHT-ANX has been helpful for some patients with promoting homework compliance.