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To make understanding and discussion possible, we have to agree on what evidence can support a claim.

Before the 17th century, people relied on authority and tradition for evidence.

After then, some people began relying on observation and experiment for evidence.

When writing a research essay or report, you should emulate a good criminal investigator: Take no one’s word for anything. Test your claims with evidence.

For inexperienced, unconvincing writers, evidence is like an ornament on a Christmas tree. For experienced, effective writers, the evidence is the trunk and branches of that tree. Without evidence, claims collapse. The essay becomes a rant, an unloaded gun getting waved around.

Four general types of evidence

Representations of reality in quantitative data

Numerical/quantified data, especially statistics

Analogies from other times and places based on quantitative data, especially statistics

In the absence of quantitative data:

Qualitative data, for example, interviews

Expert testimony

Anecdotes; hypotheticals

How evidence is expressed

Words: ink-on-paper and online

Images: still and moving (aka video)

Numbers: statistics, quantified reality

Where evidence is found

Primary sources

  • original materials on which other research is based
  • from a relevant time period
  • not filtered through interpretation or evaluation

Primary sources are usually the first formal appearance of results in physical, print or electronic format. They present original thinking, report a discovery, or share new information, especially based on observation, measurement, or surveys.

Cite primary sources whenever possible. You do not need to cite the search service or the secondary or tertiary source that you first encountered and that led you to the primary source.

Videos and images can be primary sources, especially for anecdotes and analogies.

The farther along you get in research, either academic or corporate, the primary data is more likely to be yours.

Secondary sources

  • accounts written after the primary evidence and with the benefit of hindsight
  • reports, interpretations, and evaluations of primary sources

Secondary sources like magazine and website articles are seldom evidence, but rather commentary on and discussion of evidence. Avoid quoting from them, especially if the quote itself cites primary evidence.

Videos can also be secondary sources, for example, a newscast.

Tertiary sources

  • information which collects and distills primary and secondary sources.

Tertiary sources like Wikipedia are not usually evidence but like secondary sources usually cite the primary sources. Use secondary and tertiary sources to find the primary sources.

Wikipedia does have data compilations that cannot be found elsewhere. For example, it has a sortable table of the world’s countries that includes the percentage of each country’s GDP per capita that would be earned by someone working full time at that country’s official minimum wage. It is based on data from the World Bank, but as far as I can tell, this table is original to Wikipedia and shows a computation and comparison that is not shown elsewhere. In this case, I would cite the Wikipedia page.

Adapted from a Yale University library guide, where you can learn more about sources.

Criteria for evidence

Use only valid and reliable evidence that is relevant to your claim. Use some evidence for every claim. Use enough evidence and cite the primary source whenever possible so that the reader can learn more.

Validity – Does it measure what it says it does?

Reliability – Is it evidence that further testing would confirm? Can it be generalized? If evidence is valid, it is also reliable. If evidence is reliable, it is not necessarily valid.

Relevance – Does it have a logical connection to the claim it is offered in support of?

Sufficiency – Is it enough to support the claim?