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:
Who pays for science? Does it matter? (There is evidence that it does matter)
Research may be funded by:
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 the topics discussed above
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:
An annotated bibliography is a summary of a set of articles on a given topic. It can serve as a guide for others looking for resources on that topic. Each summary, or annotation, serves as a short review of the larger paper.
For primary literature, which in science tends to be research articles, your annotation should include answers to these questions:
The main hypothesis, methods, and results should be clearly, concisely, and specifically described in a manner understandable by non-experts, and the relevance of each study for your topic should be logical and clearly described.
(Revised from: "Annotated Bibliography Assignment," UCB Dept. of Integrative Biology, 2019)