I am reviewing the urinary nicotine metabolite biomarker data and I am confused about why there are so many missing values. For example, for norcotinine, there are only 8,098 cases out of 11,519. I thought that values below the limit of detection were imputed with LOD/sqrt2, so I am wondering why participants have missing values for the biomarkers that weren't imputed. I haven't been able to find anything in the documentation or the user guide that explains why participants have missing values. Can someone please advise? The reason for missingness is going to determine whether or not we are able to use the data. Thanks!
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