Background

Novelty bias is the tendency for an intervention to appear better when it is new. It is also known as the ‘novel agent effects’ or ‘fading of reported effectiveness’.1 2 The mechanisms by which interventions appear better when new or new for a specific purpose are unknown and may involve other forms of bias having a more significant effect when an intervention is new. Novelty bias can arise when the internal or external validity is compromised. Regarding internal validity, performance bias3 and detection bias4 may cause novelty bias because unblinded researchers may be particularly enthusiastic about new treatments, leading to differences in the care received by the intervention and control groups apart from the intended treatment or differences in the outcome assessment. Selective outcome reporting bias can also be a critical reason for novelty bias.5 6 Positive result bias7 (eg, positive results of a treatment are selectively reported when it is new and less selectively reported later), confirmation bias8 (eg, only the evidence supporting the new treatments is gathered while the others are disregarded) and hot stuff bias9 (eg, researchers may be keen to confirm the positive findings regarding a new and hot topic rather than falsifying them) are examples of selective reporting bias. They can lead to overinterpretation of the point estimates …