  The mechanical stretching of single poly-proteins is an emerging tool for the
study of protein (un)folding, chemical catalysis and polymer physics at the
single molecule level. The observed processes i.e unfolding or reduction
events, are typically considered to be stochastic and by its nature are
susceptible to be censored by the finite duration of the experiment. Here we
develop a formal analytical and experimental description on the number of
observed events under various conditions of practical interest. We provide a
rule of thumb to define the experiment protocol duration. Finally we provide a
methodology to accurately estimate the number of stretched molecules based on
the number of observed unfolding events. Using this analysis on experimental
data we conclude for the first time that poly-ubiquitin binds at a random
position both to the substrate and to the pulling probe and that observing all
the existing modules is the less likely event.
