There are two different diagnostic manuals used for schizophrenias, DSM-IV and CDI-10. These are not identical suggesting they will result in inconsistant diagnosis.
These manuals also risk Proffessional Jargon. They are writted for specialists to understand not laymen, possibly causing unreliable diagnosis.
BECK- investigated the inter-rate reliablity of diagnosis between 2 psychiatrists of 154 patients. Diagnosis was only the same 54% of time, suggesting that diagnosis is only consistant 54% of the time.
For a patient to be diagnosed they must be clinally interviewed. Therefore, psychiatrists must rely on retrospective data from an individual who is unpredictable and hard to tell whether they are telling the truth due to inner voices and delusions which are characterists of shizophrenia.