Study design
This study is a randomized controlled trial. Subjects were randomized into three groups: two Internet-based self-help interventions (CBT and PST) and a waiting list control group. The study protocol, information brochure and informed consent form were approved by the Medical Ethics Committee of the VU University Medical Center (registration number 2006/168).
Inclusion and exclusion criteria
All adults aged 18 years and older with depressive symptoms who are willing to perform a self-help course, were eligible for this study. Inclusion criteria were: having elevated depressive symptoms (scoring above the cut-off score of 16 or more on the Center of Epidemiologic Studies Depression – scale (CES-D), sufficient knowledge of the Dutch language, access to Internet and having an e-mail address. No exclusion criteria were defined for this study.
Recruitment
Subjects for this study were recruited through advertisements in daily and weekly newspapers and via the Internet. This was done in a period of 5 weeks. Application took place via a website. After application, subjects received a brochure about this study and an informed consent form by post. After giving informed consent, subjects received the baseline questionnaire by e-mail. Subjects with a score of 16 or higher on the CES-D were included. Subjects with more severe symptoms of depression (indicated by a CES-D score of 32 or higher) were advised to consult their general practitioner, but could participate in this study.
Randomization
Randomization took place at an individual level after the baseline measurement and one week before the start of the interventions. Subjects were randomized into three groups – two intervention groups and a waiting list control group. Received baseline questionnaires were numbered in order of arrival. We used block randomization, with each block containing 9 allocations. An independent researcher made the allocation schedule with a computerized random number generator. A week before the start of the interventions, subjects were informed about the randomization outcome.
Interventions
Problem-Solving Treatment
Our PST- intervention is based on SET [19]. It has been expanded with more information, examples, exercises and forms. The theoretical assumption underpinning problem-solving is that psychological symptoms of depression and anxiety are often caused by practical problems people face in their daily lives. Hence, if people's problems can be resolved, their symptoms will improve. During PST, an individual learns a specific problem-solving procedure in an attempt to resolve their problems in a structured way [23].
PST in this study consisted of three steps. First the subjects described what really matters to them. Second they wrote down their current worries and problems. They categorised these problems into three categories (a) unimportant problems (problems unrelated to the things that matter to them), (b) problems which can be solved, and (c) problems which cannot be solved (e.g. the loss of a loved one). For each of these three types of problems a different strategy is proposed to solve the problems or to learn to cope with the unsolvable ones. A great amount of time is spent on solvable problems by using a six steps procedure, namely describing the problem, brainstorming solutions, choosing the best solution, making a plan for carrying out the solution, actually carrying out the solution and evaluation. During the third and last step, the subjects made a plan for the future in which they described how they will try to accomplish those things that matter most to them. The course took five weeks, with one lesson a week. Every week the participants were asked to make exercises and to send these to their coach.
Cognitive Behavioral Therapy
The Internet-based CBT intervention was developed by the Trimbos Institute, The Netherlands Institute of Mental Health and Addiction. This intervention is based on the "Coping with Depression" course (CWD) [24], Dutch version [25]. CWD is a highly structured psycho-educational form of cognitive behavior therapy for depression. Theoretically, this course is based on the social learning theory according to which depression is associated with a decrease in pleasant and an increase in unpleasant person-environment interactions. People's problems are viewed as behavioral and cognitive patterns which can be unlearned or relearned. The course was designed to provide training in skills which can be used to change these behavioral and cognitive patterns [24]. The cognitive skills are based on the cognitive therapy by Beck and his colleagues [26] and the rational emotive therapy from Ellis. The pleasant events approach has been developed by Lewinsohn and colleagues [27].
Like CWD, CBT in this study included psycho-education and focused on skills such as relaxation, cognitive restructuring (including worrying), social skills and how to increase the number of pleasant events. The intervention made use of text, exercises, audio and video fragments. CBT consisted of 8 lessons, one lesson a week. Twelve weeks later, the 9th lesson took place.
Waiting-list control group
Subjects on the waiting list received no treatment or support but started the intervention three months after the intervention group ended. Subjects on the waiting list received problem solving treatment.
Support
Subjects in both intervention groups received support during the intervention period by email from Master students in clinical psychology. Prior to the start of the interventions, they received training of 6 hours in total. This training was given by the first author of this article. First, the students read the intervention material themselves, then they carried out the assignments, and practised giving feedback to each other by e-mail. They also practised with case material and their feedback was discussed.
Support was directed at guiding the participant through the intervention, not at depressive symptoms or other problems. Every week, a standardized e-mail was sent to the participants. This e-mail communicated the lesson of that week and the date on which the assignments were to be sent to their coach. When participants sent their assignments to their coach, they received feedback within three working days. All feedback was checked by the first author and if necessary provided with comments before it was sent to the participants.
Assessments
Five assessments took place. The first assessment (baseline) took place before the start of the intervention. Other assessments took place at 5, 8, 12 weeks and 9 months after baseline.
Instruments
Primary outcome
Depressive symptoms
Symptoms of depression were measured with the Center for Epidemiological Studies Depression scale (CES-D) [28]. This questionnaire is widely used for identifying people with depressive symptomatology. Its validity has been tested in different populations [29–31]. The CES-D consists of 20 items and the total score varies between 0 and 60. Scores of 16 and higher represent a clinically significant level of depressive symptoms [28]. The cut-off score of 16 was used in this study as an inclusion criterion.
Secondary outcomes
Anxiety symptoms
The anxiety subscale of the Hospital Anxiety and Depression Scale (HADS) was used for the measurement of anxiety symptoms [32]. The anxiety subscale consists of 7 items. Scores range from 0 to 21 with higher scores indicating more anxiety. The HADS showed good homogeneity and reliability, with Cronbach's alpha ranging from .81 to .84 in different normal and clinical Dutch samples [32].
Quality of life
Quality of life was assessed with the EuroQol Questionnaire (EQ5D) [33], which is a validated tool for measuring general health-related quality of life. It consists of five items (mobility, self-care, usual activities, pain/discomfort and anxiety/depression), each of which is rated as causing 'no problems', 'some problems' or 'extreme problems'. The EQ5D thus distinguishes 486 unique health states. Each unique health state has a utility score which ranges from 0 (poor health) to 1 (perfect health). We used this single EQ5D summary index score.
Dysfunctional cognitions
The Dysfunctional Attitudes Scale (DAS) is a 40-item self-report measure designed to assess cognitive vulnerability to depression [34]. The DAS is one of the most widely used questionnaires to measure cognitions in relation to depression. The Dutch version of the DAS showed good reliability and satisfactory validity [35].
Health care utilisation
We used the Trimbos and iMTA Questionnaire on Costs Associated with Psychiatric Illness (TiC-P) [36] to collect data on direct and indirect costs from the participants. The first part of the Tic-P measures the amount of medical care received by the participants, the second part measures work productivity.
Worrying
Worrying was measured by the Penn State Worry Questionnaire (PSWQ). The PSWQ [37] is a 16-item self-report inventory. Scores range from 16 to 80 with higher scores representing more worry. De PSWQ showed good reliability in normal and clinical populations [38].
Problem-Solving skills
The Social Problem-Solving Inventory-Revised (SPSI-R) which is designed by D'Zurilla, was used for measuring problem solving skills. This questionnaire was designed to measure people's ability to resolve problems of everyday living. The SPSI-R contains 52 items and consists of the following five scales: Positive problem Orientation (PPO), Negative Problem Orientation (NPO), Rational Problem Solving (RPS), Impulsivity/Carelessness Style (ICS) and Avoidance Style (AS). Alphas for these five scales ranged from .76 to .92 and test-retest reliability ranged from .72 to .88 [39].
Perceived control
We assessed perceived control with the Mastery Scale [40]. This scale has 7 items regarding how much an individual perceives having control over things in his or her life. Items are rated on a 4-point scale with higher scores indicating greater perceived control. The questionnaire has good psychometric properties [40].
Sample size
The sample size of this study was based on the expected difference on the primary outcome variable, i.e. depressive symptoms, between the intervention groups and the waiting list control group at post-test. Based on a power of 0.80 in a one-tailed test, an alpha of 0.05, we needed 100 subjects in each condition to show an effect-size of 0.40. Therefore, the total sample size was determined at 300.
Statistical analysis
Analyses were performed according to the intention-to-treat principle. Overall treatment efficacy was performed with linear mixed modeling analysis using SPSS. General growth mixture modeling was used for the identification of subgroups in the sample and for analyzing mediating variables, using M-plus. With this method, it is possible to identify distinct groups of individuals, differing in the initial level and course of a specific behavior, through the empirical identification of developmental trajectories [41]. The advantage of this method is that the identification of subpopulations within a sample is based on the target behavior itself (in our case depressive symptoms over time). This technique also makes it possible to examine whether the effects of interventions differ for different categories of patients, to ascertain which characteristics (moderators) predict membership of one of these categories, and to establish whether outcomes are different for each category [42]. The following variables were examined for their mediating role: dysfunctional cognitions, problem solving skills, worrying, anxiety and mastery. The moderators we studied that may affect the efficacy of both treatments were: socio-demographic characteristics and severity of symptoms.