According to ggweather.com, 72-73 was a Strong Nino, 76-77 was weak, and 91-92 was strong. So you have 51-52 and 1994-95 in the last 60 years that didn't produce...but it appears the rest did. So it may not be 100%, but to me it's higher confidence enough to say it's worked much more than it hasn't!