The representative section from discoloured teeth was chosen at the most severely discoloured part of the tooth, while in non-discoloured teeth the representative section was chosen at the middle part of the tooth. - Given the relative small sample size why not use matched sections instead of using severely discolored versus middle part of tooth. Also, it appears the samples were collected from 21 different individuals? if not whats the number of individuals? also how did they know its separate individuals?