Network sampling is a sampling design in which the simple random sample or strati?ed sample of the sampling units is made and all observational units which are linked to any of the selected sampling units are taken. Different observational units might be linked to different numbers of sampling units. In the survey for estimation of the prevalence of a rare disease, for instance, a random sample of medical centers may be chosen. From the records of each medical centre in sample, records of the patients who are treated for the disease of interest could be extracted. A given patient might have been treated at more than one centre and the more centers at which the treatment has been taken, the higher the inclusion probability for the patient's records.