M. M. Slining is a coauthor of the 2010 study of human nail growth with S. Yaemsiri, N. Hou and K. He. Participants recorded displacement of nail marks over follow-up, allowing growth rate to be computed from distance and time.[ref] The record verifies this scientific collaboration while leaving the initials unexpanded.
A methodological lesson concerns averaging when observation intervals differ. Consider an original two-observation example: one mark advances 3 millimetres in one month, another 8 millimetres in two months. Their rates are 3 and 4 millimetres per month. Equal weighting of the two rates giveswhereas total displacement divided by total observed time givesThe pooled calculation weights the longer interval more heavily. Neither arithmetic error nor biological contradiction explains their difference: they answer differently weighted questions.
For a longitudinal study, the desired estimand determines the summary. An equal-person average, an average over measured nails and an elapsed-time-weighted average can differ. Measurement uncertainty and dependence among nails also affect precision. The original example illustrates interpretation of the published method; it does not attribute a particular statistical choice or individual task to Slining.
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