Depending on the fitting procedure and set of data, one may get different sets of parameters that optimally fit the data but may give very different results when extrapolating to low pressures. This is also illustrated in figure . For coal, we see that the ranking of DSL does not depend on the fitting, while for natural gas and confined spaces, we get a completely different ranking. The IAST procedure does not suffer from this and therefore yields a more reliable extrapolation to low pressures.