“which”如果使用不當可引起歧義。它常與“that”混用。“that”和“which”都引導定語從句，但“that”用于引導限定性從句，而“which”用于引導非限定性從句。

例如，“the sections that were positive for GFP were subjected to cell counting procedures”，在這個句子中，“that”引導的是限定性從句明確規定了是哪些切片用于細胞計數。相比之下，“the sections, which were positive for GFP, were subjected to cell counting procedures”，在這個句子中，對用于細胞計數的切片的規定相當寬松，可能指的是前一個句子或相臨句子中提及的切片。提及GFP陽性的從句可以向讀者提供一些額外的信息，但對于理解該句子的意義來說并非必不可少；也就是說，它是可有可無的。考慮到“which”的這種角色，研究人員在撰寫論文時應明確“which”一詞確切指代的東西——有時指代的是該詞所緊跟的事物(這是最常見的)，有時指代的則是該句子的主語。

例如，“microglia migrated to the site of the lesion, which was associated with increased levels of ED-1”，這個句子就寫得含混不清，因為我們很難確定“which”所指的到底是lesion，還是migration of microglia。如果讀者可能會對此類句子產生疑惑，最好是推翻重寫；例如，可以改為“migration of microglia to the site of the lesion was associated with increased levels of ED-1”，也可改為“microglia migrated to the site of the lesion, and immunohistochemical analysis revealed increased levels of ED-1 at this site”。兩者均無歧義。

?“Data were normalised to the housekeeping gene actin, which was used as an internal reference…” (在這個句子中,“which”指代的是actin，因此actin也就是該從句的主語)

?“Data were normalised to the internal reference housekeeping gene actin, revealing increases in the levels of…”(如指隨后從句中的分析數據,使用“which revealed”是不恰當的，且會造成歧義)