A method of deriving indoor/outdoor classification information for call data for a wireless communication network, the method comprising: identifying a set of subscriber call data records; deriving a physical channel measurement threshold value based at least partly on physical channel measurement values for the identified set of subscriber call data records, deriving the physical channel measurement threshold value including: retrieving physical channel measurement values for the identified set of subscriber call data records; determining whether a sufficient quantity of physical channel measurement values for the set of subscriber call data records is available; deriving the physical channel measurement threshold value based on a theoretical physical channel measurement value if it is determined that an insufficient quantity of physical channel measurement values for the set of subscriber call data records is available; if it is determined that a sufficient quantity of physical channel measurement values for the set of subscriber call data records is available: calculating mean and standard deviation values for the retrieved physical channel measurement values for the identified set of subscriber call data records; and deriving the physical channel measurement threshold value based at least partly on the calculated mean and standard deviation values; classifying (550, 555) subscriber call data records for the wireless communication network as indoor subscriber call data records or outdoor subscriber call data records based at least partly on a comparison of the physical channel measurement values and the derived physical channel measurement threshold value. identifying a set of subscriber call data records; deriving a physical channel measurement threshold value based at least partly on physical channel measurement values for the identified set of subscriber call data records, deriving the physical channel measurement threshold value including: retrieving physical channel measurement values for the identified set of subscriber call data records; determining whether a sufficient quantity of physical channel measurement values for the set of subscriber call data records is available; deriving the physical channel measurement threshold value based on a theoretical physical channel measurement value if it is determined that an insufficient quantity of physical channel measurement values for the set of subscriber call data records is available; if it is determined that a sufficient quantity of physical channel measurement values for the set of subscriber call data records is available: calculating mean and standard deviation values for the retrieved physical channel measurement values for the identified set of subscriber call data records; and deriving the physical channel measurement threshold value based at least partly on the calculated mean and standard deviation values; deriving the physical channel measurement threshold value including: retrieving physical channel measurement values for the identified set of subscriber call data records; determining whether a sufficient quantity of physical channel measurement values for the set of subscriber call data records is available; deriving the physical channel measurement threshold value based on a theoretical physical channel measurement value if it is determined that an insufficient quantity of physical channel measurement values for the set of subscriber call data records is available; if it is determined that a sufficient quantity of physical channel measurement values for the set of subscriber call data records is available: calculating mean and standard deviation values for the retrieved physical channel measurement values for the identified set of subscriber call data records; and deriving the physical channel measurement threshold value based at least partly on the calculated mean and standard deviation values; retrieving physical channel measurement values for the identified set of subscriber call data records; determining whether a sufficient quantity of physical channel measurement values for the set of subscriber call data records is available; deriving the physical channel measurement threshold value based on a theoretical physical channel measurement value if it is determined that an insufficient quantity of physical channel measurement values for the set of subscriber call data records is available; if it is determined that a sufficient quantity of physical channel measurement values for the set of subscriber call data records is available: calculating mean and standard deviation values for the retrieved physical channel measurement values for the identified set of subscriber call data records; and deriving the physical channel measurement threshold value based at least partly on the calculated mean and standard deviation values; calculating mean and standard deviation values for the retrieved physical channel measurement values for the identified set of subscriber call data records; and deriving the physical channel measurement threshold value based at least partly on the calculated mean and standard deviation values; classifying (550, 555) subscriber call data records for the wireless communication network as indoor subscriber call data records or outdoor subscriber call data records based at least partly on a comparison of the physical channel measurement values and the derived physical channel measurement threshold value.