Statistically significant ( P = 0.017 ) retardation of myopia progression ( 0.27 0.11 D during 24-month period or reduction ratio of 20 % ) by +1.5 D add PA-PALs relative to the SVLs was found , which was within the range of the percentage efficacy of myopia retardation by the conventional PALs in earlier trials over the same follow-up period .