In a perfectly modeled world this set of topocentric plots would show zero slope and the appropriate level of scatter. The zero slope is the consequence of projecting the TRF to each date of observation. In most cases the slopes are small and not significantly different from zero. The scatter as indicated by reduced chi-square, however, is usually statistically too large and indicates that the error budget is incomplete. The actual level of scatter is a function of both the quality of data from a site and the overall quality of the networks in which the site participated.
Some sites observe well but because of geography can only observe with weaker or distant sites.
The number of sites in a single VLBI session is small, averaging 4.3 for the entire data set and 5.3 since 1993. An underlying assumption of this type of analysis is that the sites are moving uniformly and that wild outliers are absent. The time series of these sites will show false systematic effects caused by the wandering site. Likewise, if a site's position is exceptionally discrepant on a particular day, then the time series of the other sites in that network will also show an outlier. Other sites may also show real or erroneous non-uniform behavior that may leak into their networks to a lesser degree.