The system or device may be guaranteed up to a certain period. This pe
riod is variable because of environmental factors such as vibrations,
shocks and pressures etc. This variation has an obvious bearing for th
e interest of the producer and the consumer. The present paper analyse
s the robustness of two-parameter exponential SPRT with respect to ris
ks and average sample number (ASN) when the guaranteed-time parameter
undergoes a change.