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This paper demonstrates the order of influence of end effects on the application of a theory to determine and correct the end effects in Couette coaxial cylinder rotational viscometers, which are commonly used for the rheological evaluation of drilling fluids. It is found that higher shear stresses are measured due to end effects and, consequently, this error bearing shear stress-shear rate data leads to unreliable predictions of the rheological parameters. Evaluation of the shear stress-shear rate data obtained from various drilling fluids indicates that the flow curve corrected for end effects deviates significantly from the measured curve. A parametric analysis for the rheological models, such as the Bingham plastic, power law, and yield-pseudo-plastic models, is also performed with the parameters of the selected rheological model.