Application note from Maxim Integrated about improved long term stability of their voltage references. Link here (PDF)
Large industrial plants such as those in the oil, gas, electricity, pharmaceutical, and food and beverage industries rely on electronic instrumentation to provide accurate, stable measurements to control their processes. The stability of an instrument very much depends on its application and the environment in which it operates. Fluctuating temperatures, harsh manufacturing conditions, humidity and elapsed time are all factors that affect stability.