ddkm:
Recognize that many of the compounds in the DIPPR database (or any other database) are not found in large quantities in nature. While they can be produced synthetically and purified at great expense, it can literally take weeks of intense effort just to get a few grams of material in the laboratory that is sufficiently pure for measuring properties. Also, several properties cannot be measured with very small amounts of a sample.
Therefore, it is quite common to find pure component properties that have been estimated using a very wide variety of correlations, such as group contribution methods. For many compounds, even decent critical properties cannot be measured, as the compounds decompose at temperatures way below the critical temperature.
Therefore, as Dick Russell has pointed out, one of DIPPR's objectives is to ensure that the values they "certify" for use are consistent with the best practices and procedures. Unfortunately, in the past, a lot of data has been measured with contaminated samples. That is partly what explains the wide spread in reported values. In some instances, the experimental techniques were questionable or of limited accuracy (true for many very old measurements).
There are a number of highly original contributions in the literature (e.g., by Kevin Joback and others) on estimation of properties. The definitive summary is found in Poling, Prausnitz, and O'Connell: "The Properties of Gases and Liquids" (5th edition, McGraw-Hill).
As far as the question of accuracy goes, the pertinent question in every case is: what are you trying to accomplish? For example, it would be foolhardy to insist on extremely accurate pure component vapor pressure data for mixture vapor-liquid equilibria (VLE) in those cases where you have no measured mixture phase equilibrium data (and so must rely on group contribution methods such as UNIFAC for estimating activity coefficients whose accuracy can be off by as much as 20-30%).
In general, if such a question is really critical, one must be ready to go to the literature and try to compare data from a variety of sources, including DIPPR or Yaws, and get some idea of the range of the values that have been reported. DIPPR members also get access to the original literature citations, so the effort to make such a search is rendered less laborious.
Finally, it must be remembered that going back to the laboratory for new measurements is by no means a thing of the past, although I have seen a disturbing trend in recent years to shy away from lab work. Especially for complex mixture VLE work, a few good measurements can be worth a lot more than tons of correlations.
For these reasons, personally, I do not regard any databank as sacrosanct.