OBJECTIVES: Since 2011, an early benefit assessment is required for all new drugs being launched in Germany. Evidence submitted by pharmaceutical companies in dossiers is assessed by the Institute for Quality and Efficiency in Health Care (IQWiG) and subsequently appraised by the German Federal Joint Committee (FJC). The exact determination of the patient target population plays an important role for subsequent price negotiations. In diabetes mellitus type 2 the size of target population varies considerably between dossiers.  Our aim was to explore the reasons for these differences. METHODS: We analyzed 20 dossiers with drugs for diabetes mellitus type 2 published between January 2012 and May 2015. Details regarding the estimation of the target population were extracted and compared. Based on the extractions a criteria list was developed to categorize possible reasons for different sizes of the target population. RESULTS: The estimations of the target population were mainly based on secondary data analyses of drug prescriptions. The methods and assumptions used to analyze these data varied widely. Important reasons for differences in the estimations are the kind of database, the time frame, the operationalization of diabetes patients, the specification of the target population, the type of contraindications, the consideration of currently undetected patients, the mode of extrapolation to the overall population, and the portion of statutory health insurance patients. We could not identify one reason that could explain most of the deviations in the size of the target population. Several reasons seem to interact and it was not possible to determine the direction or size of the effect. CONCLUSIONS: There is a strong need for more detailed descriptions of the methods and databases used in the dossiers to estimate the size of the target populations. A harmonization of the methods seems to be helpful to reduce the variation.