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Concept

Data saturation

The point where new interviews or observations add almost no new themes. It helps decide how many people to interview.

In Spanish
Saturación
Origin
Glaser and Strauss (1967); measured by Guest, Bunce, and Johnson (2006); in usability, Nielsen and Landauer (1993)

What problem it addresses

Answers a common question: how many people do you need to interview? Qualitative research doesn’t seek statistical representativeness: you keep interviewing until no new themes appear.

An example

In usability testing, Nielsen and Landauer calculated how many problems each additional person finds.

One person100%people≈ 31%
Finds on average about a third of the problems. Not much.
Five people100%people≈ 85%
Find around 85%. Beyond that, each additional person adds very little. Enough for one round.

Hence the usual advice: several small rounds, improving between each one, instead of a single large one.

Why it happens

  1. The first interviews uncover almost all the themes.

  2. Each additional interview repeats what you already knew.

  3. When nothing new appears, you have reached saturation.

Applications in design

  • Use it like this

    Plan an initial number and keep going while new themes keep appearing.

  • Avoid

    Using “five users” as a fixed rule for any kind of research.

  • Don’t fall for it

    If your profiles are very different from each other, you will need more people from each one.

12 of 60

What the research says

In a study with 60 interviews, almost all themes had appeared in the first 12, and the main themes in the first 6 (Guest, Bunce, and Johnson, 2006).

Debated evidence. The number depends on how homogeneous the group is and how broad the question is; it is a guideline, not a rule.

Connections

Used in

Related to

Authors and works

  1. 1967

    Barney Glaser and Anselm Strauss

    Proposed continuing to collect data until no new categories appear.

  2. 2006

    Greg Guest, Arwen Bunce, and Laura Johnson

    Measured when saturation is reached in interviews.

  3. 1993

    Jakob Nielsen and Thomas Landauer

    Calculated how many usability problems each participant finds.

Sources

Original sources

  • Glaser, B. G., & Strauss, A. L. (1967). The discovery of grounded theory: Strategies for qualitative research. Aldine.
  • Guest, G., Bunce, A., & Johnson, L. (2006). How many interviews are enough? An experiment with data saturation and variability. Field Methods, 18(1), 59–82. View source: Guest, G., Bunce, A., & Johnson, L. (2006)
  • Nielsen, J., & Landauer, T. K. (1993). A mathematical model of the finding of usability problems. In Proceedings of INTERCHI 93 (pp. 206–213). ACM. View source: Nielsen, J., & Landauer, T. K. (1993)

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