The first four tests are difficult to quantify and hence assessment of errors is not possible. Stain tests are unreliable because of other chemical moieties which may be present.
Provided solvent rubs are carried out by the same person on similar substrate with similar fabric, than an error of 2 MEK Double Rubs is not uncommon.
Pencil hardness of coating on similar substrates will normally correlate but the test is crude as an indicator for extent of cure. Pendulum and rocker hardness measurements are generally a mean of more than one experiment hence of 5% can be assured.