It is well known that the positron trapping rate obtained from positro
n lifetime measurements depends upon defect concentrations and trappin
g efficiency. If a distribution of open volume defects exists, a distr
ibution of trapping efficiency also could be present. As an example, t
he calculation of trapping efficiency as a function of radius of defec
ts in Al is presented. The correct explanation of defect size distribu
tion obtained by positron lifetime measurements needs to introduce som
e additional assumptions about trapping efficiency.