  It has been claimed that protein-protein interaction (PPI) networks are
scale-free based on the observation that the node degree sequence follows a
power law. Here we argue that these claims are likely to be based on erroneous
statistical analysis. Typically, the supporting data are presented using
frequency-degree plots. We show that such plots can be misleading, and should
correctly be replaced by rank-degree plots. We provide two PPI network examples
in which the frequency-degree plots appear linear on a log-log scale, but the
rank-degree plots demonstrate that the node degree sequence is far from a power
law. We conclude that at least these PPI networks are not scale-free.
