Sentence:  With due consideration for the limits of precision in assessing , expected rate of change in ossification of girls age 2 years , and the known variations in rate of ossification of these children as described in our preceding paper in the Supplement , each arrow with a `` shaft length '' of four <WSD>months</WSD> or less was selected as indicating `` same schedule '' at Onset and Completion , for this particular epiphysis . 
Meaning of the word marked with <WSD> tags:  a time unit of approximately 30 days