73) What are some evaluating points to Ketty's study?
-Advantage op prospective study: It has the advantage of any prospective study in that it looks at children before they show any symptoms of schizophrenia and does not rely on retrospective data. Retrospective data depend on people's memory and/or recorded data from schools, hospitals, etc., which is not always reliable. Kety and his team were able to select participants appropriately and follow them accurately because of the detailed life-long records that are kept on Danish citizens. The study was conducted over many years - this is important as schizophrenia can develop in individuals over the age of 40, even though it usually has a much earlier onset than this.
-Matching of relevant variables: The children were carefully matched on relevant variables as detailed above. It is very important to take into account factors such as socio-economic status and whether the children live in urban or rural environments. This is because low socio-economic status and urban environments are known to be risk factors for schizophrenia.
-Genes versus environment: In spite of the fact that this was a large, well-controlled study carried out over a number of years, there are some problems of interpretation. The main difficulty with family studies such as this is that they cannot differentiate between genetic and environmental influences because the children share the same environment as their mothers.
-Reliability of diagnosis: The diagnosis of schizophrenia is not always reliable. The mothers of these children had all been diagnosed with schizophrenia before the modern diagnostic systems were available, so it is possible that they varied widely in their symptoms. It might even be the case that they would not have been given this diagnosis at all if later criteria had been used. People diagnosed with schizotypical personality disorder were also included in the overall figures, whereas this is different from schizophrenia and may have different causes. In fact, in DSM-IV-TR, it is regarded as a distinct personality disorder rather that a schizophrenic spectrum disorder.