Are you talking about joint efficiency being:
0,70 for non Xray tested welded joints
0,85 for spot tested welded joints
1,00 for fully tested welded joints?

Joint efficiency is a factor included into the formula for calculating the vessel wall thickness. If the welded joint has been fully Xray tested, it won't have any defects and the thickness can be smaller.
If it hasn't been Xray tested at all, you don't know whether there are defects in the welded joint, and so the thickness is greater to compensate for the possible defects that may exist, even if the joint has been made by a qualified welder. 
If it has been partially Xray tested, it's an intermediate situation and so is the joint efficiency (0.85 is the mean value between 0.70 and 1.00).
In certain cases, however, ASME VIII requires that the welded joint be fully radiographed, as for example when the vessel will contain lethal gases.
Now, if you're talking of another type of joint efficiency, please let us know.

Giovanni S. Crisi
Sao Paulo - Brazil