The study design of national surveys of periodontal disease often uses partial‐mouth examination protocols to rationalize the need for resources so that the collection of data from a large number of subjects can be achieved within the available means without significantly sacrificing precision and validity. Studies show that surveys that use partial examination protocols underestimate the prevalence of periodontitis, and the amount of underestimation varies depending on the number and type of sites examined, the case‐definition of periodontal disease, tooth loss, and prevalence and severity of the disease. If a survey uses a partial‐mouth examination it is recommended that the amount of underestimation be assessed in the same sample. This could be achieved by performing a full‐mouth examination on a randomly selected subsample. Inflation factors should be calculated and used to adjust for the underestimation in disease prevalence because of the use of partial examination. Based on the NHANES III (1988 to 1994) data and adjusting for the bias caused by the examination protocol, it is estimated that approximately half of the United States population aged ≥30 years has periodontitis.