[TheCoelacanth]

Next turbine to fail means you sample based on time points, so you still could have sample bias.

Say that turbines have an average lifespan of X years, and from year 0 to 10 you bought 90% Siemens and then from year 10 to 20 you bought 10% Siemens and then you measure failure rates from year X to year X+10.

Based on that data you would predict that Siemens turbines will be the most likely to fail next, but they are probably actually less likely to fail because most of the ones that are likely to fail soon are already gone.