I think this is not a good method to predict the quality of the ADC.
Also it is essential how you have connected the circuitry to the part!
1. the temperature drift is mainly influenced by the reference used - internal Vref, external Vref - external voltage source or reference resistor
2. on-chip reference is worst case 20 ppm/°C = 24µV/°C - to improve this you can use a external better reference
3. unless you use specific ultra low TC, accurate, but very expensive resistors as reference you see also the drift from that
For normal application you need to use calibration in software to compensate for drift.
So in the first step I suggest you use a external voltage from an calibrator as a source for the ADC input and repeat the measurements to see the accuracy of the part.
If this is not accurate enough, you can use a better external reference.
Then you can do tests with a resistor to predict the performance of your resistor and circuitry.
But please also read this http://www.analog.com/static/imported-files/rarely_asked_questions/moreInfo_raq_resistors.html