Table 6.

Quality classification and definition of grade evidence

Quality level	Definition
High (a)	Very sure that the true effect value is close to the effect value estimation
Medium (b)	There is a moderate degree of confidence in the value of effect; the real effect value may be close to the estimated value, but there is still a possibility that the two are not the same
Low (c)	There is limited confidence in the effect estimates; the true effect values may not be the same as the effect estimates
Very low (d)	There is little confidence in the estimated effect; the true effect value may be quite different from the effect estimate