What causes disease?
How you conceptualize your topic affects how you search for relevant information.
Consider first perhaps the interaction of environmental factors (eg, pollution, outbreaks) and social factors (eg, smoking, drug use). You may also wish to consider other aspects of your topic.
Think about the wider context of your topic. Do some preliminary exploration, both in the literature and in discussions with your teachers, advisors, and peers. What are the relevant scientific and policy circumstances?
Is your topic researchable?
You may need to broaden or narrow the focus of your topic.
This may become more apparent as you search for and find information. It may prove difficult to find research on very narrow topics, or to cope with the vast literature on an un-focused, broad topic.
What is the scope of your search?
Literature searching always involves balancing finding all relevant citations (which means you may also find many non-relevant citations) with finding only relevant citations (which means you may miss some relevant citations).
The search scope, as well as the purpose and audience of your literature search, influences how you focus your search when using online databases, as well as when you decide you have "enough."
Remember that research is not a linear process; you may find yourself modifying your search terms as you explore your topic.
Which disciplines are concerned with your topic? Which aspect(s) - legal, political, environmental, behavioral etc. - of your topic is/are of interest?
Answering these questions will help you decide which databases to search for literature. Although PubMed may be the best place to start for public health topics, you may miss key literature if you do not use other resources.
What to consider as you find articles and other documents:
What is evidence?
All research is (potentially) "evidence" and there are no "perfect" studies.
Critically evaluating what you read will help any unearth biases or methodological shortcomings that may be present
Is there an agenda (bias)?
It's doubtful that any study of humans is without some kind of bias, either in the study design, or in the author's pre-existing beliefs. How bias in methodology was controlled and the significance of bias in any particular study is what's relevant.
Things to consider:
This article (PDF) discusses the "manufactured uncertainty" created by industry groups that sponsor research and publishing on chemicals.
Is qualitative research "evidence"?
» If your goal is to understand beliefs and meanings in the group with whom you are working, then qualitative studies can be important.
» Take a look at a few citations (.doc) on these topics
Reliability and validity
Reliable data collection: relatively free from "measurement error."
» Is the survey written at a reading level too high for the people completing it?
» If I measure something today, then measure it again tomorrow using the same scale, will it vary? Why?
Validity refers to how well a measure assesses what it claims to measure
» If the survey is supposed to measure "quality of life," how is that concept defined? Is it measurable?
(Adopted from Chapter 3, Conducting research literature reviews : from the Internet to paper, by Arlene Fink; Sage, 2010.)
Extensive discussions of reliability and validity are available in several texts, such as Textbook in Psychiatric Epidemiology (3rd Ed.; M. Tsuang et al. Wiley. 2011; See chapters 5 and 7).
What to consider when looking at survey or estimated data:
Critical Appraisal Tools
Critical Appraisal of Intervention Studies is a free online learning module from Canada's National Collaborating Centre for Methods and Tools. It demonstrates how to assess the quality of an intervention study and to develop skill in applying the criteria for critical appraisal of an intervention study to enable you to determine whether that intervention can be applied to your own public health situation.