Sentence:  With each of the above methods, the quartile values are always either one of the data points, or exactly half way between two data points. Those methods involve only simple arithmetic and are easily extendable to octiles (eighths), <WSD>hexadeciles</WSD> (sixteenths), etc. They are not, however, extendable to quintiles (fifths) or percentiles (hundredths), etc. Furthermore, they tend to have a high bias. (That is, the quartile values calculated on subsets of the data set tend to vary more, and are not good predictors of the quartile values of the entire data set.) 
Meaning of the word marked with <WSD> tags:  Any of the quantiles which divide an ordered sample population into sixteen equally numerous subsets; a subset thus obtained.