Concordance Rate is a term that is used in statistics and by geneticists to describe the rate of probability that two people with shared genes will develop the same organic disease. As many conditions, i.e. schizophrenia and cancer, are only partially related to genetics with some components of the disease process being due to external factors (environment, lifestyle, etc).
These conditions may or may not occur despite a genetic predisposition. Statistically, the chance of two identical twins both being diagnosed with schizophrenia is estimated to be around 50%, and fraternal (non-identical) twins being around 15%.