How many interviews are enough? An experiment with data saturation and variability. Guest, G., Bunce, A., & Johnson, L. Field methods, 18(1):59–82, 2006.
bibtex   
@article{guest_how_2006,
	title = {How many interviews are enough? {An} experiment with data saturation and variability},
	volume = {18},
	issn = {1525-822X},
	number = {1},
	journal = {Field methods},
	author = {Guest, Greg and Bunce, Arwen and Johnson, Laura},
	year = {2006},
	pages = {59--82},
}

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