  A remarkable similarity between the large-scale non-Gaussian pattern of
cosmic microwave background (CMB) temperatures obtained by Wilkinson Microwave
Anisotropy Probe (WMAP) mission and the distribution feature of observation
numbers is noted. Motivated from such a similarity, in this work we check the
WMAP data for the correlation between pixel temperature t and observation
number N. Systematic effect of imbalance differential observation and
significant t-N correlation in magnitude, distribution non-Gaussianity and
north-south asymmetry are found. Our results indicate that, for precision
cosmology study based on WMAP observations, the observation effect on released
WMAP temperature maps has to be further carefully studied.
