One of the integral elements in defining success of a focus group is careful recruitment of participants. The researcher needs to ensure that certain people are kept out of the group.
Firstly, unless otherwise explicitly stated, we need to keep out close friends, siblings, colleagues etc of selected respondents. They are likely to mirror each other’s views, taking valuable group time away from meaningful participation by others.
The people participating in groups must be reasonably physically & mentally normal, so that they are able to actively contribute to the group, rather than being ‘cared for’ by others or their opinions getting acceptance from others.
Children should not be allowed to tag along with mothers, nor doting parents in a children’s group.
Professional respondents must be kept out to avoid spoiling the group completely. If it is a smartphone-specific focus group, we need to leave out people working in such industry like employees, retailer, distributors and sales promoters. Even those who are from such families should be kept out because they are likely to be very different in their attitudes and behavior from typical respondents.
Specialists, experts such as doctors, teachers, consultants in that specific product/field under discussion should be kept out, again because of their higher knowledge on the subject and likelihood of rational behavior and informed opinions/choices.