Clinical psychodiagnostics is based on theory development of the complaints and problem behaviour (1), operationalization and measurement (2) and the application of relevant diagnostic methods (3).

There are five steps in the diagnostic process:

1. Converting the provisional theory into concrete hypotheses.
2. Selecting a specific set of research tools.
3. Making predictions about the results or outcomes from this set of tools.
4. Applying and processing instruments.
5. Give reasons for why the hypotheses have been accepted or rejected.

There are five basic questions that form the basis of most questions that are posed in diagnostic process:

1. Recognition
   These questions involve the question of what the problems are. It involves inventory and description (1), organization and categorization (2) and examination of the seriousness of the problems (3).
2. Explanation
   These questions involve the question of why the problems exist. It involves the main problem (1), the conditions that explains the problem’s occurrence (2) and the causal relationship between point one and two (3).
3. Prediction
   These questions involve the question of how the problems will develop. A prediction pertains to a relation between a predictor and a criterion.
4. Indication
   These questions involve the question of how the problems can be resolved. This step does not necessarily involve selection.
5. Evaluation
   These questions involve the question of whether the problems have been resolved as the result of an intervention. This establishes whether the therapy took account of the diagnosis and treatment proposal (1) and whether the treatment has brought about a change in the client’s behaviour (2).

Classification refers to assigning the clinical picture to a class of problems. Diagnostic formulation focuses on the individual and one’s unique individual clinical picture. Explanations can be classified according to different things:

1. Locus
   In these explanations, the explanatory factor lies within a person (i.e. person-focused explanations) or within the situation (i.e. situation-focused explanations).
2. Nature of control
   This determines whether the explanation is characterized by causes (i.e. objective facts) or reasons (i.e. voluntary actions). This is a continuum.
3. Synchronous and diachronous explanatory conditions
   The synchronous explanatory conditions (i.e. current problems) coincide with behaviour that is to be explained at the time. Diachronous explanatory conditions (i.e. past problems) precede this behaviour.
4. Induced and persistent conditions
   The induced conditions give rise to a behavioural problem while persistent conditions perpetuate the behavioural problem.

There are three elements of indication:

1. Knowledge of treatment and therapists
   There needs to be knowledge of treatment and therapists, although the eligibility requirements for treatments and therapists are often not clear.
2. Knowledge of the relative usefulness of treatments
   There needs to be knowledge of the relative usefulness of treatments in order to assess whether the client would benefit from it.
3. Knowledge of the client’s acceptance of indication
   There needs to be knowledge of the likelihood that the client will or will not follow the recommendation made. Therefore, the client’s preferred treatment needs to be known.

Analysis of the request for diagnosis includes information about the referrer, which can provide information about the seriousness and the setting. The analysis aims to understand the type and content of the request and the analysis is supported by what the referrer already knows about the client.

The analysis of the request is followed by a reflection phase in which weight is given to each piece of information. However, adding different degrees of importance to pieces of information does not always improve the prediction.

In a diagnostic scenario, the diagnostician organizes all of the requester’s and the client’s questions from the application phase. On the basis of this information, the diagnostician proposes an initial, tentative theory about the client’s problematic behaviour. This scenario is used to determine which information should be assigned to the recognition question and what should be examined as the explanation.

The diagnostic examination consists of several steps:

1. Hypothesis formulation
   In this step, the diagnostician formulates a number of hypotheses based on the diagnostic scenario. The predictive hypotheses are based on empirical knowledge of successful predictors.
2. Selection of examination tools
   The selection of examination tools is determined by the nature of the question (1), the psychometric quality of the instruments (2) and the efficiency considerations (3). It is guided by the hypotheses.
3. Formulation of testable predictions
   In this step, the criteria against which the client will be examined needs to be established.
4. Administration and scoring
   In this step, the examination tools are administered to the client and the score of the test is interpreted.
5. Argumentation
   In this step, the administration and scoring are linked back to the hypotheses and predictions and the decision regarding the hypotheses is made and substantiated.
6. Report
   In this step, a report is made which contains the results of the diagnostic examination.

Diagnosis and treatment combinations (DTCs) states that once a patient is efficiently diagnosed, the protocol-based treatment that then takes place is evidence-based and clearly fits the diagnosis that has been made. There are several critiques of DTCs:

1. Many clients experience multiple problems, which makes it difficult to choose one treatment regarding diagnosis.
2. Some clients do not have clearly defined complaints making a referral problematic.
3. Within a department, there is a strong focus on one type of problematic behaviour, leading to the possibility that other important existing disorders are overlooked.
4. DTCs assume an interaction between a specific diagnosis and a type of treatment which is not always supported by empirical research.
5. There often is insufficient time to carry out a comprehensive diagnostic examination of the causes of the problems which makes selecting the right treatment difficult.