Quality and entropy depend on position in the sequence. A. Quantile-quantile plot of fast-q quality score against the information content per base. The two measures are loosely correlated, but clearly not equivalent. B. Boxplot of the fast-q score along the first 35 bases of the sequencing. The overall base quality decreases sharply after base 14, but the distribution still extends up to the top 40 score at bases 30–35. C. Frequency of the four categories of ambiguous IUPAC codes as a function of the position in the sequence.