Evidence can be quantitative or qualitative:
- Quantitative evidence is based on data that can be put into numbers. This includes quantitative measurements and statistics that can be analysed with respect to past outcomes, and used to build models to predict future outcomes. The data are often generated by carefully designed comparisons of outcomes in 2 or more controlled systems. This is the preferred evidence in disciplines such as science, medicine and economics. Claims made in these fields usually require meaningful quantitative supporting evidence to be credible.
- Qualitative evidence is based on data that can be put into words, such as narratives or comparisons based on observations and descriptions. This includes identifying themes across a set of events or observations, and seeing where there may be connections or commonalities. In some fields of research, such as social science, comparative quantitative studies are difficult to do, and qualitative studies such as surveys or case studies provide the most robust form of evidence.
- Qualitative examples are supporting pieces of information, such as quotes and extracts from relevant texts, experts and other research. Claims made in a humanities, arts or similar context require supporting information of this sort to be persuasive.
In some fields, mixed methods research combines elements from qualitative and quantitative methods to provide evidence. For example, a researcher might conduct a multiple-choice survey (quantitative) and then ask for further comments in free-form boxes or interview people in person about their responses (qualitative).
The table shows the types of evidence that can be considered for quantitative and qualitative research, and the supporting information that should be provided.
Examples of types of evidence and key information to include when writing about such evidence
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Methodology
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Examples of evidence and supporting information for various fields
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Quantitative
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Primary evidence:
- experimental data
- observed data (measurement or scoring with human observation or using instruments)
Secondary evidence:
- official or public data and statistics (archives, records, census data, maps, contracts)
- reviews and reports
Supporting information:
- details of data, including
- context (experimental or observational)
- sample size
- sample method (eg random, stratified, volunteer)
- statistical significance of calculated statistics
- discussion of dependent and independent variables, carefully defined and differentiated
- discussion of correlation, and clear articulation of significance and possible causation
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Qualitative
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Primary evidence:
- direct observations (in a real-world or experimental context)
- surveys and interviews (groups or individuals)
- transcripts; audio or video recordings
- personal items and artefacts (eg photographs, objects, letters, art works)
- eyewitness accounts, autobiographies, diaries, records kept by agencies or other organisations (eg speeches, meeting minutes)
- contemporaneous accounts (eg newspaper reports)
Secondary evidence:
- journal articles
- histories and biographies
- textbooks
- documentaries
Supporting information:
- identification of primary vs secondary evidence
- identification of selection method for sample group from which data were collected (eg random, stratified, volunteer)
- explanation of factors that may influence information provided, such as
- language ability: can subjects express themselves fully? Are documents translated from the original?
- environmental context: are subjects likely to be candid? Is groupthink a factor?
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