Once adequate reliability has been achieved, however, it is important to document internal consistency and external validity. This chapter has summarized some of our work with the former topic. It suggests that the SANS has high internal consistency, while the SAPS is somewhat less internally consistent. What are the clinical and research implications of these results concerning internal consistency? On a superficial and statistical level, these results might be considered to suggest that the SAPS is less valid than the SANS, and perhaps that therefore it is not useful. On the other hand, it might be argued that it is not necessary to measure all the items on the SANS, since they are highly intercorrelated with one another and any one might 'stand in place' for the others. While in a sense statistically correct, these conclusions are probably misleading in both clinical and research settings. The SANS and SAPS were designed primarily as descriptive instruments that are useful for encoding symptoms commonly observed in psychiatric patients. Essentially, these results document clinical common sense. Patients with one negative symptom tend to have several others, while patients with one positive symptom do not necessarily have others. In other words, affective flattening seems to be related to alogia and avolition, but delusions and hallucinations do not necessarily occur together. In a comprehensive description of an individual patient, it is important to document all the types of symptoms that are present. This is particularly crucial in pharmacologic studies, where one may wish to document that some symptoms or groups of symptoms are more responsive to treatment than are other symptoms. For example, although anhedonia and alogia are statistically correlated with one another in a population of schizophrenics, in an individual patient anhedonia might be more responsive to treatment with a specific medication than is alogia. In spite of the high intercorrelations, it remains important in clinical settings to evaluate all relevant symptoms. The results of the factor analyses in this second study do not suggest as clean and strong a separation between positive and negative symptoms as was indicated in our original study. Factor analysis is notoriously sample-dependent, but there is no reason to suspect that the sample in the second study was different in any way from that of the first. Both involved consecutive admissions of DSM-III schizophrenics to the Iowa Psychiatric Hospital. The individuals doing the clinical evaluation did change, however.(ABSTRACT TRUNCATED AT 400 WORDS)