There are several major principles of youth interventions:

1. Children and young people rarely refer themselves
2. Children are rarely treated in isolation
3. There is a chance to intervene early
4. There is almost always a lot of heterogeneity
5. Development has to be taken into account

Children not referring themselves leads to problems with client motivation and problem awareness. There are three levels of youth intervention:

1. Universal (i.e. targets whole population)
2. Selective (i.e. targets youth with certain risk)
3. Indicated (i.e. targets youth already showing symptoms).

Treatment refers to targeting youth with symptoms at (sub)clinical level or those with a diagnosis. Heterogeneity in complaints exist because of three reasons:

• There is ongoing development in youth and this makes the complaints more likely to cause problems and more heterogeneous.
• There are more people involved and this makes complaints more heterogeneous.
• The DSM criteria are less clear in youth.

The question is always where one should intervene when a child presents with complaints. Efficacy refers to whether an intervention works in a controlled, laboratory setting. Effectiveness refers to whether an intervention works in clinical practice. An evidence-based intervention refers to an intervention where there is at least some evidence regarding the efficacy and effectiveness. However, there is no clear guideline for when something is evidence-based as it is a continuum. Most include at least two studies with the following:

• Careful specification of the patient population.
• Random assignment of participants to conditions.
• Use of treatment manuals that document the procedures.
• Multiple outcome measures including a measure for the disorder.
• Statistically significant differences between treatment and comparison group after treatment.
• Replication of outcome effects.

There are several ways in which a treatment can be considered well-established:

1. Superiority to placebo or other treatment.
2. Equivalence to already established treatment.

Or alternatively, when a large series of single-case designs have been used, it must demonstrate efficacy with:

3. A good use of experimental design.
4. Comparison of intervention to another treatment.

When an experiment is used, it must be conducted with treatment manuals (1), characteristics of the sample must be specified (2) and the effects must be demonstrated by at least two different teams (3).

An intervention is probably efficacious when there are at least two experiments that show that the treatment is superior to the waiting list control-group or when the treatment is well-established but has not been investigated by at least two teams. Alternatively, a treatment is probably efficacious when a small series of single-case design experiments meet well-established treatment criteria. An intervention is an experimental treatment when treatment has not been tested in trials meeting criteria for methodology.

Evidence-based practice refers to using evidence-based interventions and using available knowledge about the effectiveness and efficiency of a method in carrying out an action. It is important to be critical of current knowledge.

There are several issues in research:

• Studies with different characteristics are combined into one effect size in meta-analyses.
• There is high heterogeneity and high comorbidity of depression and this influences the effectiveness of treatment.
• A lot of randomized controlled trials have low power.
• It is often not clear what works best for children.
• It is not clear whether recurrent depression should be treated differently.
• It is not clear what approaches work outside of standard protocols.

Studies typically demonstrate whether treatment works but is not clear in for whom it works and how it works. Treatment mediators regard the underlying mechanisms of change and is tested in intervention trials. This is important to explain to clients and the therapy trajectory may change because of the theory (e.g. “how does this treatment work for my client?”). Treatment moderators regard which treatment works for which client and this is based on research outcomes and clinical experience. Knowledge of treatment moderators can allow for personalized treatment. Treatment predictors refer to pre-treatment variables which have a main effect on treatment outcome but no interactive effect. It can predict the response in different treatment groups but the effect size of the treatment is the same regardless of the value of the target measure. It predicts change across treatment conditions. Iatrogenic effects refer to mechanisms that lead to worse treatment outcomes.

Almost one third of children being treated do not improve. Finding mediators, moderators or predictors could help these children. Moderated mediation refers to a mediator that differs according to a moderator variable. A reciprocal mediation model refers to a reciprocal causation between the mediator and the treatment condition. A sequential mediation model refers to two or more mediators intervening in a series between treatment condition and outcomes (e.g. treatment -> mediator 1 -> mediator 2 -> outcome). Including multiple mediators allows a study to evaluate the specificity of mediational effects (i.e. variables only mediating for a specific treatment).

There are three types of moderators:

1. Match-to-intervention moderator
   This includes baseline characteristics and this could match a group to an intervention.
2. Contextual moderators
   This refers to contextual characteristics (e.g. environmental stressors) which make a person fit a specific treatment.
3. Generalizability moderators
   This refers to characteristics that allow to assess whether interventions are robust to clinical complexity and whether interventions generalize to multiple treatment groups.

Common moderators or predictors are sociodemographic information (1), parent characteristics (2), family characteristics (3), child characteristics (4), school-related factors (5) and aspects of the disorder itself (6). Comorbidity does not predict or moderate immediate treatment outcomes but may predict long-term outcomes.

There are several common mediators:

• Parental characteristics (e.g. parenting practices).
• Child characteristics (e.g. emotion regulation).
• Family characteristics (e.g. cohesion).
• School functioning (e.g. relationship with teacher).

Specific factors refer to processes aimed to be changed by an active treatment (e.g. avoidance behaviour). Non-specific factors refer to characteristics that are shared by most treatments (e.g. therapeutic alliance). Both factors should be tested as mediators.