38.5% and 42.2% of the drivers of passenger cars consider fuel
price as “important” and “very important” respectively. In contrast,
the majority of freight vehicle drivers (FVD) (50%) only
choose “neutral” on this option. The difference of fuel price
between these two groups (0.566) is larger than that of any other
attribute.

在一篇论文里面看到这么一段话。我想问这里0.566是什么指标，也就是什么指标能反应他这里说的两组之间的差异