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Primary Sources

                red line            What is a primary source?

Primary sources are original materials, such as reports on original research, ideas, or scientific discoveries and results, findings, or data from experiments or research studies. They are generally written by the person or group who performed the research or experiment. 

Examples of primary sources

  • Experimental data
  • Laboratory notes
  • Conference Proceedings and Papers
  • Technical Reports
  • Patents
  • Correspondence
  • Dissertations or Theses
  • Interviews
  • Studies or Surveys

Example of a primary source from a database: 

Perkin MR, Logan K, Tseng A, Raji B, Ayis S, Peacock J, Brough H, Marrs T, Radulovic S, Craven, J, et al. 2016. Randomized trial of introduction of allergenic foods in breast-fed Infants. N Engl J Med. 374(18):1733-1743. doi:10.1056/NEJMoa1514210


Background: The age at which allergenic foods should be introduced into the diet of breast-fed infants is uncertain. We evaluated whether the early introduction of allergenic foods in the diet of breast-fed infants would protect against the development of food allergy.

Methods: We recruited, from the general population, 1303 exclusively breast-fed infants who were 3 months of age and randomly assigned them to the early introduction of six allergenic foods (peanut, cooked egg, cow's milk, sesame, whitefish, and wheat; early-introduction group) or to the current practice recommended in the United Kingdom of exclusive breast-feeding to approximately 6 months of age (standard-introduction group). The primary outcome was food allergy to one or more of the six foods between 1 year and 3 years of age.

Results: In the intention-to-treat analysis, food allergy to one or more of the six intervention foods developed in 7.1% of the participants in the standard-introduction group (42 of 595 participants) and in 5.6% of those in the early-introduction group (32 of 567) (P=0.32). In the per-protocol analysis, the prevalence of any food allergy was significantly lower in the early-introduction group than in the standard-introduction group (2.4% vs. 7.3%, P=0.01), as was the prevalence of peanut allergy (0% vs. 2.5%, P=0.003) and egg allergy (1.4% vs. 5.5%, P=0.009); there were no significant effects with respect to milk, sesame, fish, or wheat. The consumption of 2 g per week of peanut or egg-white protein was associated with a significantly lower prevalence of these respective allergies than was less consumption. The early introduction of all six foods was not easily achieved but was safe.

Conclusions: The trial did not show the efficacy of early introduction of allergenic foods in an intention-to-treat analysis. Further analysis raised the question of whether the prevention of food allergy by means of early introduction of multiple allergenic foods was dose-dependent. (Funded by the Food Standards Agency and others; EAT Current Controlled Trials number, ISRCTN14254740.).

Secondary Sources

                red line            What is a secondary source?

Also known as review articles, secondary sources summarize, interpret, or analyze the research and discoveries of other people. Generally, the purpose of secondary sources is to familiarize readers with a topic.

Examples of secondary sources: (some of these could also be considered tertiary sources)

  • Criticism and Interpretation, public opinion, moral and ethical aspects or social policies
  • Dictionaries or encyclopedias
  • Handbooks, textbooks, guides to literature
  • Government Policy or Law and Legislation
  • Reviews or review articles
  • Tables

Example of a secondary resource from a database:

Sanduleanu S, Woodruff HC, de Jong EEC, van Timmeren J, Jochems A, Dubois L, and Philippe L. 2018. Tracking tumor biology with radiomics: a systematic review utilizing a radiomics quality score. Radiother Oncol. 127(3):349-360. doi:10.1016/j.radonc.2018.03.033


Introduction: In this review we describe recent developments in the field of radiomics along with current relevant literature linking it to tumor biology. We furthermore explore the methodologic quality of these studies with our in-house radiomics quality scoring (RQS) tool. Finally, we offer our vision on necessary future steps for the development of stable radiomic features and their links to tumor biology.

Methods: Two authors (S.S. and H.W.) independently performed a thorough systematic literature search and outcome extraction to identify relevant studies published in MEDLINE/PubMed (National Center for Biotechnology Information, NCBI), EMBASE (Ovid) and Web of Science (WoS). Two authors (S.S, H.W) separately and two authors (J.v.T and E.d.J) concordantly scored the articles for their methodology and analyses according to the previously published radiomics quality score (RQS).

Results: In summary, a total of 655 records were identified till 25-09-2017 based on the previously specified search terms, from which n = 236 in MEDLINE/PubMed, n = 215 in EMBASE and n = 204 from Web of Science. After determining full article availability and reading the available articles, a total of n = 41 studies were included in the systematic review. The RQS scoring resulted in some discrepancies between the reviewers, e.g. reviewer H.W scored 4 studies ≥50%, reviewer S.S scored 3 studies ≥50% while reviewers J.v.T and E.d.J scored 1 study ≥50%. Up to nine studies were given a quality score of 0%. The majority of studies were scored below 50%.

Discussion: In this study, we performed a systematic literature search linking radiomics to tumor biology. All but two studies (n = 39) revealed that radiomic features derived from ultrasound, CT, PET and/or MR are significantly associated with one or several specific tumor biologic substrates, from somatic mutation status to tumor histopathologic grading and metabolism. Considerable inter-observer differences were found with regard to RQS scoring, while important questions were raised concerning the interpretability of the outcome of such scores.

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