When summarizing my sources methodology for my literature review, must I add a citation? My professor told me to critically summarize and evaluate each work (for its premise, methodology, and conclusion) I am including in my Literature Review. I understand that I should provide in-text citations when mentioning the source; however, I'm not sure that I should write an in-text citation for every sentence describing how the study was conducted.

Ex: A study by Lina Aldén and Mats Hammarstedt examined cohorts of refugees who migrated to Sweden between the years 2005 and 2007, tracking them longitudinally up until 2012. Data used in the study was obtained from the LISA database at Statistics Sweden, which included information about refugees national origin, age, gender, educational attainment, number of children, employment status, and taxable income, all of which are influential in determining migrants economic contribution. The results from their analyses reveal that the average refugee had public sector net costs of SEK 142,250 (13,714€) during the first year of residency (Citation). After seven years, this cost was halved (Citation).

I added citations to the last two sentences where I mention the results of the study, but I am confused as to whether I need to add citations following each sentence describing the study. This will be in MLA if that's relevant to the question.
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.