Most polls conducted today use telephone interviewing. Interviewers are not usually permitted to vary the question in any way, and no extemporaneous explanations are allowed. In-home interviews can also be conducted but these are more expensive. Surveys can be conducted by mail or on-line, though there are concerns that samples will not be representative (although for targeted populations with an incentive to reply, these methods can be efficient and cost effective). For telephone surveys, the usual method is random digit dialing (RDD) or use of the most recently published telephone listings. If using the latter method, one usually chooses telephone numbers randomly, but then changes the last digit to ensure that unlisted numbers and those who recently got telephones have an equal chance of being selected.
Cluster sampling is usually used for in-home interviewing, whereby representative clusters in particular neighbourhoods are selected (the costs of travelling hundreds of kilometres to conduct one interview make a truly random sample impossible for in-home interviews). Quota sampling can also be used. ...
For example, to be able to make reasonable extrapolations about, say, Atlantic Canadians, in a survey, it may be necessary to "oversample" Atlantic Canada. However, when looking at the national results, each Atlantic Canadian respondent would be counted as less than a full respondent, otherwise Atlantic Canadians would be overrepresented in the sample. This is what "weighting the data" means, and it is customary to weight the data by region, gender and age to ensure representivity. Polling firms now tend to use the cati system (computer assisted telephone interviewing) in which the interview process is streamlined. Answers are automatically submitted into a data bank, question filters can be used that ask different respondents different questions depending on their previous answers, and the wording of questions can be randomly altered.
Understanding error: margin and otherwise
We have all heard poll results in news stories described as being, "accurate to plus or minus three percentage points, 19 times out of 20," but what does this mean' This statement, and the figures that it contains, refers to the "sampling error" (3%) and "confidence interval" (95%, or 19 out of 20) of the poll that has been taken. This means that 95 percent of all samples taken from the same population using the same question at the same time will be +/- the sampling error (usually referred to as the "margin of error"). The reported margin of error assumes two things: that the sample was properly collected (and therefore represents the cross-section of the target population), and that the questions are properly designed and in fact measure the underlying concepts that are of interest. If either of these two