Hello Dr. Frank,
There are three methods for testing drift, the 'sitting on a shelf' at room temperature with periodic measurements, works great but takes a long time and also has increasing uncertainty in the readings over time if you are splitting hairs. The next method is putting the resistors in an oven at a specified elevated temperature over a period of time and taking periodic readings, this takes less time but costs more to do, has the advantage of faster results and minimal changes in uncertainies. The third method, which is what was used with my resistors, is the thermal shock testing, it is more expensive but provides results in a few hours and the uncertainty factor is essentially the same as when the testing began.
As I mentioned elsewhere, a third party subjected 50 resistors to 50 thermal cycles (+125°C to -55°C, 30 minutes, with no more than 2 minutes between temperature, well beyond the normal 5 cycles, the resistors were found to be superior in stability to every other resistor they had previously tested (even at 5 cycles) and with a zero failure rate. I can state that most resistors had the equivalent drift of <2 PPM/year, but I still specify 5 PPM as some units can be a little higher. It was also found in studies conducted by institutes of standards (Australians for one), that the Evanohm alloy (of which all of mine are made from or a derivative) has by far the best long term stability of any resistance alloy IF USED PROPERLY. The actual design of the resistor (i.e. construction) has a very significant bearing on how well the resistor performs, while it is not impossible for other resistor houses to produce 0 ±3 PPM/°C resistors (check their specs, only over a limited range in most cases), their stability is significantly worse because of their construction. It has been and still is believed by virtually all other resistor houses that they produce a welded, stable PWW resistor, when if fact, their own specifications prove otherwise.