1. A computerized method of developing a model to estimate disability insurance exposure arising from insuring a group of individuals to be underwritten for a coverage period comprising: providing a computer access to a medical insurance claim database comprising data for a plurality of medical insurance claims made by members of a first model population, the number of members eligible to make medical insurance claims, the data for a medical insurance claim comprising at least a medical condition code relating to a medical condition of a first model population member, with each first model population member not necessarily having any corresponding medical insurance claims; providing a computer access to a disability insurance claim database comprising data for a plurality of disability insurance claims made by members of a second model population, which may partially overlap with the first model population, the data for a disability insurance claim comprising at least a medical condition code regarding a cause of disability, with each second model population member having at least one disability insurance claim; providing the computer a morbidity category classification routine that categorizes medical condition codes into morbidity categories, the morbidity categories grouping medical condition codes that are generally homogeneous with respect to incidence of disability arising from those underlying medical condition codes that are grouped into the morbidity category; processing the medical insurance claim database for each member of the first model population with the morbidity category classification routine to obtain a database of morbidity categories for the medical insurance claims of each of the members of the first model population; processing the disability insurance claim database for each member of the second model population with the morbidity category classification routine to obtain a database of morbidity categories for the disability insurance claims of each of members of the second model population; providing the computer a prevalence rate calculation routine; processing the database of morbidity categories for the medical insurance claims of each of members of the first model population and the number of members eligible to make medical insurance claims, with the prevalence rate calculation routine to obtain a database of prevalence rates of each morbidity category within the members of the first model population; providing the computer a proportion calculation routine; processing the database of morbidity categories for the disability insurance claims of each of members of the second model population with the proportion calculation routine to obtain a database of values for the proportion of the second model population that each morbidity category is present in; providing a database of the probability of disability arising in members of a third population not related to morbidity category; providing the computer a morbidity category conditional probability calculation routine, the morbidity category conditional probability routine implementing a conditional probability estimation method to provide a probability of disability arising for a morbidity category given the prevalence value of the morbidity category within the members of the first model population, the value for the proportion of a second model population that the morbidity category is present in, and the probability of disability arising in members of a third population not related to morbidity category; and processing, with the morbidity category conditional probability calculation routine, for each of the morbidity categories, the database of prevalence rates of each morbidity category within the members of the first model population, the database of values for the proportion of the second model population that each morbidity category is present in, and the database of the probability of disability arising in members of a third population not related to morbidity category to obtain a database of probabilities of disability for each of the morbidity categories; wherein the database of probabilities of disability for each of the morbidity categories comprises a model that can estimate disability exposure arising from insuring a group of individuals to be underwritten for a coverage period by applying the model medical insurance claim codes of the members of the group of individuals.