It is difficult in that every clothing line and pattern maker
have different ideas of what measurement belongs to
what size. And with finished clothing, I've found that most
of the time, the more expensive the line, the larger the old
sizes run. For instance, in a garment from Sears or such,
you might buy a size 14, but the same measurements in a
very expensive garment might claim to be an 8 or 10!!
It's called vanity sizing, which has nothing to do with reality.
Like the others said, measure three times and cut once.