In this paper the results of eddy current inspections of pitting in heat exchanger tubes are used. For the residual lifetime estimation two methods are used. The first method is based on a linear corrosion rate and uses a Gaussian distribution of the corrosion. Whereas the second uses a Weibull distribution. This kind of distribution makes it possible to use any kind of corrosion rate including non-linear. In order to obtain a thorough estimation both methods take possible errors and the number of inspected tubes into account. While gathering more and more inspection results and adapting each method it became obvious that the Weibull model is in favour for lifetime prediction. As a first result of the analysis by means of sub- sampling it became obvious that for a condition determination with an accuracy of 0.1 mm an inspection of 100 tubes is sufficient. But it also showed that for a prognoses of the condition for the future with the same reliability of 0.1 mm more than 1000 tubes should be inspected.