There are an increasing number of ways to evaluate a measurement system, and each of these is intended to answer a different question about the suitability of the system.
If your question is "Is this gauge adequate to inspect a part to this tolerance?", then P/T ratio (known as % R&R before the proliferation of indicators) answers this question. The typical standard for P/T ratio is 10% or less is ideal, between 10% to 30% may be acceptable depending on the importance of the measurement, the cost of new gauging, etc., and > 30% is not acceptable.
If your question is "Is this gauge adequate to inspect a part as part of study (e.g., Six Sigma, DOE, capability, etc.)?", then %TV (Total Variation) answers this question. The standard is the same as above. In addition, the Number of Destinct Categories is used. An ndc >/= 5 is good, between 2 & 5 may be acceptable, and < 2 is unacceptable. Wheeler uses a Discrimination Ratio, which must be >/= 4, and requires the gauge resolution to be < the StdDev of PV (Part Variation).
If you want to know where the variation is coming from, % Contribution of each component answers this question.
See AIAG's MSA manual and/or Wheeler's "Evaluating the Measurement Process" for more information.