Age related changes in accommodative dynamics in humans: Response to Dr. Schachar’s letter

Dear Editor, We are writing to respond to Dr. Schachar's letter (Schachar, 2007) regarding our study (Kasthurirangan & Glasser, 2006). The issues raised by Schachar are either founded on improper analysis or are already addressed in the article. Schachar has made assertions regarding analysis from just three subjects. In the original paper, trends in the changes in accommodative dynamics with age were demonstrated in a group of 66 subjects. It is inappropriate and obtuse to attempt to extrapolate general trends on the age related changes in accommodative dynamics in 66 subjects ranging in age from 14 to 45 from just 3 individual subjects. 1. and 2. Regarding our age subgroup analysis, in the paper we state: ''An a priori decision was made to include five subjects in each of the young and old groups with a small range of ages within each group and a wide age range between the two groups.'' This subgroup analysis illustrates the specific changes in accommodative dynamics with age and is consistent with general trends based on the entire study group. The difference in the mean age between the two groups is 17.25 years and the difference in the predicted accommodative amplitude is 4.5 D. The range of ages (2.92 years) and the range of predicted accommodative amplitude (0.76 D) in the older subgroup represent only 17% of the difference between the groups. If anything, the larger age range in the older subgroup would tend to increase data scatter and obscure the differences between young and old groups. In spite of this, clear differences can be seen between the two subgroups in Fig. 8. Therefore, the difference in age within the older group is small compared to the overall difference, and clear trends are seen despite any potential confound. Finally, the reviewers of the original manuscript expressed no concerns with this analysis. Since these two subgroups were not arbitrarily selected, we can make no statements regarding how selection of arbitrary subgroups influences the results, nor does arbitrary selection of different subgroups, as Schachar has done, serve any value. 3. In fact, as shown in Fig. 8B clear differences (in fact, statistically significant differences) can be seen between young and old subjects for any amplitudes above about 0.5 D. Therefore, the statement that clear differences in dynamics between young and old subjects can be seen for all response amplitudes above 0.5 D (Figs. 8A and B) …