Sorry - you are really talking out of your rear end here ...

7 deployments (gawd knows how many ppl carry them and how many trips where may be needed - but I'd guess it is in the millions)
of those 7, 2 were succesful - that is 28.5% chance of success. 4 were only partially successful - ie a failure - 56% only partially succeesful then ... therefore 14.25% failure ...

as I said - 7 deployments doesn't make for good statistics - the sample size is far too small to be of any use. The figures given do not give any other variables - make/size/age/last service so you cannot use any of these figures.