The reported prevalence of TS varies "according to the source, age, and sex of the sample; the ascertainment procedures; and diagnostic system", with a range reported between .4% and 3.8% for children ages 5 to 18. Robertson (2011) says that 1% of school-age children have Tourettes. According to Lombroso and Scahill (2008), the emerging consensus is that .1 to 1% of children have Tourettes, with several studies supporting a tighter range of .6 to .8%. Bloch and Leckman (2009) and Swain (2007) report a range of prevalence in children of .4 to .6%, Knight et al. (2012) estimate .77% in children, and Du et al. (2010) report that 1 to 3% of Western school-age children have Tourettes.
Question: How many years after Du et al.'s report came out was the Knight et al. report released?
Answer: 2