



 
Hi Ammara, The method for calculating percentage change is described in our response to an earlier question. For the old lenses I think that in the paper you are reading they take the earlier measurement to be 2.26 and the later to be 3.28 so the calculation is \[\frac{3.28  2.26}{2.26} = \frac{1.02}{2.26} = 0.4513 \mbox{ or } 45.13 \%.\] For the new lens the calculation gives \[\frac{2.75  2.24}{2.26} = \frac{0.51}{2.26} = 0.2377 \mbox{ or } 23.77 \%.\] Since 23.77% is approximately one half of 45.13% I think the authors feel they can say that "the new lenses 'slowed the progression of refractive error by approximately 50%'". I hope this helps, 



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