This study examines how the same online consumer ratings can be perceived differently by consumers in different purchasing circumstances and demonstrates that the effects of ratings proportionally vary with scale and assessment types. By applying Weber’s law, which explains the shifting perception of individuals on stimuli based on mental scaling mechanism, we hypothesize that the marginal effect of rating (i.e. when there is one product to value) and rating difference (i.e., when there are two products to compare) decrease as rating and rating difference increase. To validate this hypothesis, we conducted an experimental survey on 226 online shopping mall users. The results show the marginally decreasing effect of rating and rating difference on consumer purchase intention, thus supporting the different mental scaling systems in different situations. This study contributes to the literature by providing rationale for consideration of the scale and context of the rating when discussing the rating effect.