What are the best predictors of a worker's achieving good attenuation from an ear plug?

 One-on-one training has the most significant impact a good earplug fit.

Answer Extra: 

 In a recent study of factors that influence a good earplug fit, there were no personal factors that were accurate predictors of a good fit of an ear plug in the field: Gender, age, years in noise, ear canal size, familiarity with product, and the model of earplug were simply not good predictors of whether a user would achieve good attenuation.

When program factors were evaluated to predict a good ear plug fit, there was one factor that stood out distinctly with a strong correlation: one-on-one training. That is, the more often a worker had received individual training in the proper use of hearing protectors, the higher the probability of a good fit. The same could not be said for group training; it appeared to make no difference at all whether a worker had attended zero, five, or 10 group training sessions in hearing protection, when measuring good attenuation in the field. (There are other good reasons to offer group training in a Hearing Conservation Program, but group training is not the best method for teaching hearing protector fitting procedures.)

Enforcement was also a good predictor of good earplug performance, but only when it was coupled with one-on-one training. When strong enforcement policies were in place without proper training, workers were actually more likely to have a poor fit than a good fit.

Howard Leight Acoustical Laboratory. "Assessing Fit Effectiveness of Earplugs." http://www.hearforever.org/tools-to-learn/assessing-fit-effectiveness-of-earplugs