1 out of 1 or 2 out of 3?

Today many organisations are going in for two out of three redundant logic systems for trips. I often wonder how I managed to operate an ammonia plant 25 years ago, fitted with standalone pneumatic instrumentation and no DCS! With modern day electronics, isn't it expected that the reliability of an electronic transmitter will be better? Do not go in for two out of three transmitters just because it is more "reliable". Have you obtained data on mean time between failures of electronic transmitters? When you do a LOPA analysis do not go overboard. The risk criteria used in LOPA should reflect your organizations past incidents also. I have seen many overkills of LOPA studies done by consultants who just recommend two out of three systems at the drop of a hat!