When summarizing my sources methodology for my literature review, must I add a citation?
The answer to the question in the title is "yes, always". In your example text I would simplify to 


  Aldén and Hammarstedt (2008) examined ...


This only works in the author-year notation though, not the numerical or footnote-style. In those cases (I have no experience writing in those) I would probably say


  A study by Lina Aldén and Mats Hammarstedt (Citation) examined ...


and then use a full paragraph to set off this part from the rest of the text. This implies that there is a connecting topic in the paragraph and the early citation makes it clear what the topic is.