Many in-vivo diagnostic devices provide measurements on a continuous scale and thus precision of these devices as part of measurement validation is an important aspect of the device evaluation. Experimental designs, factors, and mathematical models to evaluate repeatability, intermediate reproducibility and reproducibility for such devices will be discussed. The CLSI documents EP05-A3 and the ISO documents 5725 discuss various designs for laboratory tests which can lend to the studies for in-vivo diagnostic devices.