An important issue that should not be confused with validity is precision.
Precision is a measure of the likelihood of chance effects leading to random errors.
It is reflected in the confidence interval around the estimate of effect from each study and the weight given to the results of each study when an overall estimate of effect or weighted average is derived. More precise results are given more weight. Variation in validity can explain variation in the results of the studies included in a systematic review. More rigorous studies may be more likely to yield results that are closer to the 'truth'. It is important to systematically complete critical appraisal of all studies in a review even if there is no variability in either the validity or results of the included studies.