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The classic book by Shadish, Cook, and Campbell makes a useful distinction here.

http://www.amazon.com/Experimental-Quasi-Experimental-Designs-Generalized-Inference/dp/0395615569

They divide what most people call "external validity" into two: 1) Testing what they call external validity means asking if the effect of the same treatment is the same in a different time or place, 2) testing what they call "construct validity" means asking if the effect is the same when the treatment itself is different (perhaps in the same setting).

The Tamil Nadu Integrated Nutrition Project is a perfect example of the measured effect differing because the treatment bundle is not the same in the two experiments---a difference of construct validity that could occur even in the same time-and-place.