1. A method for generating test data for at least one particular field of a record format describing two or more fields, the method performed by one or more computer systems and including: reading values occurring in a field of multiple records from a first dataset, the records each including multiple fields, the field from which the values are read corresponding to the at least one particular field for which the test data is to be generated, and the records each including the field corresponding to the at least one particular field for which the test data is to be generated; storing profile information for the at least one particular field, the profile information including a list identifying, for two or more of the values read from the field in the multiple records, information about the frequency at which each of the two or more values read from the field in the multiple records occurs in the field in the multiple records; generating a model of a probability distribution for the at least one particular field, based on the list; based on the model of the probability distribution, generating a second dataset including multiple test data values for the at least one particular field such that a frequency at which a given value read from the field in the multiple records occurs in the test data values in the second dataset corresponds to a probability assigned to that given value in the first dataset by the model; and storing the test data including the test data values in a data storage system.