  Test set with redundancy is one of the focuses in recent bioinformatics
research. Set cover greedy algorithm (SGA for short) is a commonly used
algorithm for test set with redundancy. This paper proves that the
approximation ratio of SGA can be $(2-\frac{1}{2r})\ln n+{3/2}\ln r+O(\ln\ln
n)$ by using the potential function technique. This result is better than the
approximation ratio $2\ln n$ which directly derives from set multicover, when
$r=o(\frac{\ln n}{\ln\ln n})$, and is an extension of the approximability
results for plain test set.
