1. A method for use in performing underwriting for a group of individuals to be underwritten for a coverage period comprising: providing an underwriting computer access to a database of probabilities of disability and durations of disability for each of a plurality of morbidity categories created by: providing the modeling 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 the modeling 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 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 and a duration of disability, with each second model population member having at least one disability insurance claim; providing the modeling 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 modeling 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 modeling 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 the modeling computer a duration calculation routine; processing the database of morbidity categories for the disability insurance claims of each of members of the second model population with the duration calculation routine to obtain a database of values for the duration of disability for each morbidity category; providing the modeling computer a database of the probability of disability arising in members of a third population not related to morbidity category; providing the modeling 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; providing an underwriting computer access to an underwriting medical insurance claim database comprising data for a plurality of medical insurance claims made by a group of individuals, the number of individuals 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 one of the individuals, with each individual not necessarily having any corresponding medical insurance claims; providing the underwriting computer an underwriting morbidity category classification routine that is consistent with the morbidity categories of the database of probabilities of disability for each of a plurality of morbidity categories provided by the modeling computer; processing the underwriting medical insurance claim database for each individual of the group with the underwriting morbidity category classification routine to obtain a database of morbidity categories for the medical insurance claims of each of the individuals of the group; providing the underwriting computer with a routine that applies to each individual's morbidity categories the probabilities of disability from the database of probabilities of disability for each of a plurality of morbidity categories to generate a probability of disability for that individual; providing the underwriting computer with a database on disability compensation rates for the individuals in the group; providing the underwriting computer with coverage duration limitation information reflecting the terms of a policy for the group to be underwritten; providing the underwriting computer with an individual disability exposure compensation routine that combines an individual's probability of disability, duration of disability and disability compensation rate to calculate an estimated cost of disability for that individual while limiting the duration of disability in accordance with the duration limitation information; processing the database of probabilities of disability, the database of disability durations and the database of disability compensation rates to create a database of expected costs of disability for the individuals of the group; providing the underwriting computer with an individual cost summing routine that totals the expected costs of disability of the individual members of the group; and processing each individual's expected costs of disability with the cost summing routine to generate a total expected cost of disability for the group wherein all the processing steps are performed by a computer.