A common mistake that people make when first writing a literature review is to present the literature in a neutral and even way, as if it is all equivalent.
‘Smith (2009) found that pigs can fly. Jones’ study (2009) suggested that this is not the case.’
This doesn’t help your reader to understand or contextualise this information (for example, Smith may have been looking at airline travel for animals, while Jones was studying aerodynamics). More importantly, it doesn’t help your reader to understand your position. Will your paper / thesis take the position that pigs can or can’t fly?
Ambiguity also occurs if you give an author’s name at the beginning of a sentence, because this often implies you don’t have a position on what they say. For example.
‘Smith (2009) says the moon is made of green cheese’.
It’s clear what Smith thinks, but it’s not clear what you think.
However, if you say:
‘The moon is made of green cheese (Smith, 2009)’
you are making it clear that you think this is a fact, which was first uncovered by Smith.
The other option is to keep the author at the beginning, but only because you are going to disagree with their view.
Smith (2009) argued that the moon is made of green cheese but subsequent observations (Jones, 2012) have shown that it is mainly composed of dog biscuits.’
Here you are showing that you are aware of Smith’s view, but that you agree with Jones.