neglects the principles of probability samples, increasingly requires from the statistical profession to specify the conditions where non-probability samples may work. These issues, and potential remedies, are discussed in a number of sources. Telephone Use of interviewers encourages sample persons to respond, leading to higher response rates. Interviewers can increase comprehension of questions by answering respondents' questions. Fairly cost efficient, depending on local call charge structure Good for large national (or international) sampling frames Some potential for interviewer bias (e.g., some people may be more willing to discuss a sensitive issue with a female interviewer than with a male one) Cannot be used for non-audio information (graphics, demonstrations, taste/smell samples) Three types: Traditional telephone interviews Computer assisted telephone dialing Computer assisted telephone interviewing (CATI) Mail The questionnaire may be handed to the respondents or mailed to them, but in all cases they are returned to the researcher via mail. An advantage is, is that cost is very low, since bulk postage is cheap in most countries Long delays, often several months, before the surveys are returned and statistical analysis can begin Not suitable for issues that may require clarification Respondents can answer at their own convenience (allowing them to break up long surveys; also useful if they need to check records to answer a question) No interviewer bias Non-response bias can be corrected by extrapolation across waves Large amount of information can be obtained: some mail surveys are as long as 50 pages Response rates can be improved by using mail panels Response rates can be improved by using prepaid monetary incentives Response rates are affected by the class of mail through which the survey was sent Members of the panel have agreed to participate Panels can be used in longitudinal designs where the same respondents are surveyed several times Face-to-face Suitable for locations where telephone or mail are not developed Potential for interviewer bias Easy to manipulate by completing multiple times to skew results Mixed-mode surveys Researchers can combine several above methods for the data collection. For example, researchers can invite shoppers at malls, and send willing participants questionnaires by emails. With the introduction of computers to the survey process, survey mode now includes combinations of different approaches or mixed-mode designs. Some of the most common methods are: Computer-assisted personal interviewing (CAPI): The computer displays the questions on screen, the interviewer reads them to the respondent, and then enters