  In pulsar timing, timing residuals are the differences between the observed
times of arrival and the predictions from the timing model. A comprehensive
timing model will produce featureless residuals, which are presumably composed
of dominating noise and weak physical effects excluded from the timing model
(e.g. gravitational waves). In order to apply the optimal statistical methods
for detecting the weak gravitational wave signals, we need to know the
statistical properties of the noise components in the residuals. In this paper
we utilize a variety of non-parametric statistical tests to analyze the
whiteness and Gaussianity of the North American Nanohertz Observatory for
Gravitational Waves (NANOGrav) 5-year timing data which are obtained from the
Arecibo Observatory and the Green Bank Telescope from 2005 to 2010 (Demorest et
al. 2013). We find that most of the data are consistent with white noise; Many
data deviate from Gaussianity at different levels, nevertheless, removing
outliers in some pulsars will mitigate the deviations.
