Language must be used accurately - the words used to describe behaviour should be accurate and appropriate and must have valid operationalised definitions
Researcher bias must be avoided - e.g. it's not okay to make notes only on events that support the researcher's theories, or to have a biased interpretation of what is observed
When closed questions are used as part of an interview's structure, quantitative data can be produced by counting the number of participants who replied 'Yes' to a particular question
The situation in which a participant says something, and the way they are behaving at the time, may be important. It may help the researcher understand why something is said, and give clues about the honesty of a statement.
The interviewer should be aware of how their feelings about the interviewee could lead to biased interpretations of what they say or how it is later reported
It's especially important to distinguish the interpretations of the researcher from the statements of the participant, and to be unbiased in selecting what to include in any report on the research
The analysis of written answers may be especially difficult because the participant is not present to clarify any ambiguities, plus you don't know the context for their answers (e.g. what mood they were in, and so on)
A way to quantify qualitative data by analysing the data into categories or typologies (e.g. sarcastic remarks, statements about feelings, etc.), quotations, summaries, and so on
Meta-analyses can reduce the problem of sample size, but there are often loads of conflicting results out there, which makes doing a meta-analysis a bit tricky