The parameters of the life length distribution of a given component ar
e to be estimated. The observations on which inference is to be based
are field data which are incomplete in some fashion. Thus, for example
, the reported life length may include a period of unknown duration du
ring which the component is not in use, the life length distribution m
ay be affected by an unobserved environmental factor or the component
may be part of a larger system, and failure mode analysis reveals only
the module containing the failed component, not its identity. It is s
hown how the EM algorithm can be used to calculate the maximum likelih
ood estimates of the parameters of interest in these instances. The me
thodology is applied to some data on the life lengths of electronic co
mponents used in the telecommunications industry, yielding values that
are similar to those obtained from complete observations on comparabl
e components.