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Google instantly pointed to a Martin Bland page http://www-users.york.ac.uk/~mb55/intro/intent.htm with a worked medical example. More generally, the terms seem to refer to a population and the basis for assuming that selection was random. In the human context this may be interpreted as volunteer bias (http://www.psychwiki.com/wiki/What_is_volunteer_bias%3F) and avoidance - both upsetting the "missing at random" assumption. In animal sampling the terms are less appropriate, but there are well-known examples. In insects where males have wings and females do not, you clearly need different sampling techniques. Many diseases cause infected animals to become listless and not feed, so less likely to be caught in traps. On the other hand, some parasites affect the nervous system and cause affected animals to expose themselves to predation. If only life was like an urn with fixed number of black and white balls to sample ... Allan -----Original Message----- From: A UK-based worldwide e-mail broadcast system mailing list [mailto:[log in to unmask]] On Behalf Of Stephen Senn Sent: 27 June 2011 15:31 To: [log in to unmask] Subject: Re: Volunteer bias / non-response bias Your assessment seems makes sense to me. I would say that the one describes the individuals you see and the other those you don't but they are, as you say, two sides of the same coin. Stephen Stephen Senn Professor of Statistics School of Mathematics and Statistics Direct line: +44 (0)141 330 5141 Fax: +44 (0)141 330 4814 Private Webpage: http://www.senns.demon.co.uk/home.html University of Glasgow 15 University Gardens Glasgow G12 8QW The University of Glasgow, charity number SC004401 ________________________________________ From: A UK-based worldwide e-mail broadcast system mailing list [[log in to unmask]] On Behalf Of Dr Philip Sedgwick [[log in to unmask]] Sent: 27 June 2011 15:29 To: [log in to unmask] Subject: Volunteer bias / non-response bias Dear fellow Allstaters Was wondering if anyone has an opinion on the definitions of volunteer bias and non-response bias, especially in context of questionnaire surveys. There is some potential overlap but I do not believe that one is a simple inverse of the other. Any thoughts Best wishes Philip Sedgwick __________________________________________ Dr. Philip Sedgwick Senior Lecturer in Medical Statistics, Academic Lead and Chief Examiner for SSCs and Projects Section of Medical and Healthcare Education, Room 6.30, Sixth Floor, Hunter Wing St. George's, University of London, London SW17 0RE Telephone +44 20 8725 5551 Fax +44 20 8725 3584 Email [log in to unmask] Twitter: http://twitter.com/#!/Statistics_Man You may leave the list at any time by sending the command SIGNOFF allstat to [log in to unmask], leaving the subject line blank. You may leave the list at any time by sending the command SIGNOFF allstat to [log in to unmask], leaving the subject line blank. This email and any attachments are intended for the named recipient only. Its unauthorised use, distribution, disclosure, storage or copying is not permitted. If you have received it in error, please destroy all copies and notify the sender. In messages of a non-business nature, the views and opinions expressed are the author's own and do not necessarily reflect those of Cefas. Communications on Cefas’ computer systems may be monitored and/or recorded to secure the effective operation of the system and for other lawful purposes. You may leave the list at any time by sending the command SIGNOFF allstat to [log in to unmask], leaving the subject line blank.