A correct diagnosis will allow the clinician to describe base rates, causes and treatment. Reliability and validity are the cornerstones of any diagnostic or assessment procedure. Reliability refers to the consistency of measurement. Validity refers to measuring what you want to measure. Alternate form reliability refers to the extent to which scores on two forms of the test are consistent. Criterion validity is evaluated by determining whether a measure is associated expectedly with some other measure. If both variables are being measured at the same time, it is concurrent validity.

The diagnostic and statistical manual of mental disorders (DSM) is the diagnostic system used by many mental health professionals. Specific diagnostic criteria (1) and extensively described characteristics of diagnosis (2) were added to the DSM-III and have been retained ever since.

There are some major changes of the DSM into the DSM-5:

1. Removal of the multiaxial system
2. Organizing diagnoses by causes
   There are no proper tests to organize diagnoses around aetiology, so the diagnosis is based around symptoms. The chapters in DSM-5 are organized to reflect patterns of comorbidity and shared aetiology.
3. Enhanced sensitivity to the developmental nature of psychopathology
   Across diagnoses, more detail is provided about the expression of symptoms in younger populations.
4. New diagnoses
   New diagnoses were added to the DSM-5.
5. Combining diagnoses
6. Ethnic and cultural considerations in diagnosis
   There are many different cultural influences on the risk factors for psychological disorders, the types of symptoms experienced, the willingness to seek help and the treatments available.

There are some criticisms of the DSM:

1. Too many diagnoses
   A side effect of the huge number of diagnostic categories is comorbidity. Different diagnoses do not seem to be distinct in their aetiology or treatment and this can lead to too many diagnoses.
2. Categorical classification versus dimensional classification
   Categorical classification refers to putting people in categories (e.g: disorder “yes” or “no”). It could be useful to use dimensional classification, describing the degree to which a disorder is present. Advantages of categorical classification are being more certain on when to offer treatment.
3. Reliability of the DSM
   The reliability of the DSM has to be good for the DSM to be useful. The DSM is not always reliable.
4. Validity of the DSM
   The DSM is not always very valid.

Diagnosing someone can have the disadvantages of changing a person’s ability to function and stigmatize a person. Diagnosis can lead us to focus on the disorder and ignore important differences among people.

There are several methods in which psychological assessment can be obtained.

Mental health professionals can use formal and structures as well as informal and fewer structures clinical interviews for psychological assessment. In a clinical interview, the interviewer pays attention to how the respondent answers questions. Trust is imperative for psychological treatment. A structured interview can be used, in which the questions are set out in a prescribed fashion for the interviewer. If clinicians use an informal interview, instead of a structured interview, the reliability of diagnoses tends to be lower.

Measuring stress is important for psychological assessment. Stress is the subjective experience of distress in response to perceived environmental problems. Stress can be measured by the Bedford College Live Events and Difficulties Schedule (LEDS). It is a semi-structured interview that covers a lot of stressors. LEDS is used to evaluate the importance of any event in the context of a person’s life circumstances and to exclude events that might be consequences of symptoms. Life stressors are predictors of psychological disorders. A self-report stress checklist can also be used to assess stress levels. A problem with self-report measures of stress is that people vary in how they look at stressors and that people might forget certain stressors.

Psychological tests are also for psychological assessment. There are two types of personality tests:

1. Self-report personality inventories
   This is a questionnaire with habitual tendencies and a person has to determine whether these apply to the person.
2. Projective personality test
   A projective test is a psychological assessment tool in which a set of standard stimuli ambiguous enough to allow variation in responses is presented to the person (e.g: inkblots). The assumption is that the person’s responses will be determined by true attitudes, motivations and modes of behaviour.

Intelligence tests are also used for psychological assessment and can be used to predict school performance (1), diagnose learning disorders (2), help assess whether a person has an intellectual disability (3), identify intellectually gifted children (4) and be used as part of neurological evaluations (e.g: dementia) (5). IQ is correlated with health and risk for mental disorders.

There are also assessment methods that focus on behavioural and cognitive characteristics. It includes the following: aspects of the environment that might contribute to symptoms (1), characteristics of the person (2), the frequency and form of problematic behaviour (3) and consequences of problem behaviours (4). This can be done in several ways:

1. Direct observation of behaviour
2. Self-observation
   This includes self-monitoring. It can include ecological momentary assessment (EMA). With EMA, a person receives a signal several times a day and asked to enter responses in a device. Reactivity, the process wherein behaviour changes because it is being observed, occurs with self-observation as well as with other forms of observation of behaviour.

Cognitive-style questionnaires can also be used.

CT and MRI scans reveal the structure of the brain. PET reveals brain function and to a lesser extent, brain structure. fMRI is used to assess both brain structure and function. In post-mortem studies, the brains of deceased people are removed and the number of specific neurotransmitters in particular brain areas can then be directly measured. In alive participants, one method of neurotransmitter assessment involves analysing the metabolites of neurotransmitters that have been broken down by enzymes. Metabolites are acids produced when a neurotransmitter is deactivated. It can be drawn from bodily fluids or for a more specific measure, drawn from a person’s spinal cord.

A neurologist is a physician who specializes in diseases or problems that affect the nervous system. A neuropsychologist is a psychologist who studies how dysfunctions of the brain affect the way we think, feel and behave. Neuropsychological tests are often used in conjunction with the brain-imaging techniques just described. Psychophysiology is concerned with the bodily changes that are associated with psychological events.

Cultural bias in assessment refers to the notion that a measure developed for one culture or ethnic group may not be equally reliable and valid with a different cultural or ethnic group. This bias can be overcome when conducting assessments by being aware of the cultural differences and how it can potentially influence assessment. Cultural awareness is important.