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In order for systematic reviews to make accurate inferences concerning clinical therapy, the primary studies that constitute the review must provide valid results. The Cochrane Handbook for Systematic Reviews states that assessment of validity is an “essential component” of a review that “should influence the analysis, interpretation, and conclusions of the review.” The internal validity of a review’s primary studies must be considered to ensure that bias has not compromised the results, leading to inaccurate estimates of summary effect sizes.   In ophthalmology, there is a need for closer examination of the validity of primary studies comprising a review. As an illustrative example, Chakrabarti et al (2012) discuss emerging ophthalmic treatments for proliferative and nonproliferative diabetic retinopathy. In the article, the author states that anti-VEGF agents consistently receive the most interest as possible alternative treatments for diabetic retinopathy. The article provides that guidelines set by the Scottish Intercollegiate Guidelines Network and American Academy of Ophthalmology mention anti-VEGF treatment as merely \textit{useful as an adjunct} to laser for treatment of PDR, however, the Malaysian guidelines indicate that these same agents were \textit{to be considered in combination} with intraocular steroids and vitrectomy. Most extensively, the National Health and Medical Research Council guidelines \textit{recommend the addition} of anti-VEGF to laser therapy prior to vitrectomy (2012). Eldaly et al (2014) conducted a systematic review of this anti-VEGF treatment for diabetic retinopathy. In this review, 18 randomized controlled trials were included in the final assessment. Of these 18 RCTs, seven were at high risk of bias while the rest were unclear in one or more domains (2014). The author goes on to conclude that, “there as very low or low quality evidence from RCTs for the efficacy and safety of anti-VEGF agents when used to treat PDR over and above current standard treatments. However, the results suggest that anti-VEGFs can reduce the risk of intraocular bleeding in people with PDR.” By including studies with very low and low quality in a review, it is difficulty for clinicians to make a sound judgement when it comes to disease treatment.  Validity and quality have been very heavily researched areas, particularly in recent years. Validity has been described as the ability of the instrument to measure what it is believed it is measuring (Moher 1995). Researchers have used many different methods to attempt to evaluate the validity and quality of primary studies. Initially, checklists and scales were developed to evaluate whether particular quality items, such as randomization, blinding, allocation concealment, etc., were addressed in the study. Although these are effective at evaluating specific components of study validity, they are often denounced for falsely elevating quality scores. Many of these scales and checklists include items that have no bearing on a study’s actual quality, such as whether there was informed consent or whether there was ethical approval (1995). Furthermore, it was suggested that the choice of scale or checklist could alter the results of systematic reviews (Jüni 1999), and because of this, two main tools emerged as the most accurate: the Jadad scale (Jadad 1996) and the Downs and Black checklist (Downs 1998). Reporting guidelines developed by the predominant review board at this time, QUORUM, necessitated the evaluation of methodological quality of the primary studies in systematic reviews. The Cochrane Collaboration decided there was a need for a new tool, and in 2008 it developed the Cochrane Risk of Bias Tool. Its development was based on a combination of empirical and theoretical considerations, leading to a focus on risk of bias rather than study 'quality' and a division of assessments into six bias domains (Stern 2013). The risk of bias was then ranked as low, high, or unclear with no score calculation. Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA), which provides the updated reporting guidelines, now calls for the evaluation of bias in all systematic reviews (Moher 2009).  
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