For a bit more background on early ETOPs discussions see the latter part of post #26 in Tech Log (
http://www.pprune.org/9180842-post26.html) in the thread on "savings from fewer engines".
I won't repeat it all here but in the 1982 ICAO ETOPS study group, the statistical data put forward by "the industry" (Boeing/Airbus/IATA) was very flaky. They did not want many issues that result in engines and systems not delivering their full performance to be counted. They were strongly opposed by IFALPA and others, which led to their opening position being rejected and the much more detailed rules that today allow very long range diversions.