Each article that has passed the screening process for a systematic review should be assessed for risk of bias. The Cochrane manual defines risk of bias as "a systematic error, or deviation from the truth, in results." It further states that "Biases can lead to under-estimation or over-estimation of the true intervention effect and can vary in magnitude: some are small (and trivial compared with the observed effect) and some are substantial (so that an apparent finding may be due entirely to bias)." (Cochrane Manual: https://training.cochrane.org/handbook/current/chapter-07)
The resources listed below can aid in determining risk of bias scores for studies included in your systematic review.