A specific form of selection bias is an observation selection effect, or anthropic bias. That is, for some reason you've only observed certain evidence, so your conclusions are based around what you're able to see. You fail to notice and take into account what you don't see - that much is obvious when stated like that, but how many people think "I have never seen X, therefore X does not exist". X, of course, being something that others have observed, or at least something potentially visible such as in the Black Swan problem. It's important not to confuse it with absence of evidence. gnostic 10:35, 18 January 2012 (UTC)
Truman, Dewey and the phone
Wasn't the infamous poll an example of selection bias - it was a phone poll when phones were less common and more likely to be owned by Dewey supporters? 188.8.131.52 (talk) 22:09, 25 December 2016 (UTC)
- Neither TOW's article nor these two .edu sources mention phones, afaik. I don't know that these sources are in any way exhaustive, but they're what came up after a 5 second look on Google.
- In fact, the first .edu source I link above specifically inform us that no phones were used:
For the 1948 election between Thomas Dewey and Harry Truman, Gallup conducted a poll with a sample size of about 3250. Each individual in the sample was inteviewed in person by a professional interviewer to minimize nonresponse bias, and each interviewer was given a very detailed set of quotas to meet. For example, an interviewer could have been given the following quotas: seven white males under 40 living in a rural area, five black males under 40 living in a rural area, six black females under 40 living in a rural area, etc. Other than meeting these quotas the ultimate choice of who was interviewed was left to each interviewer.
Based on the results of this poll, Gallup predicted a victory for Dewey, the Republican candidate.
- According to the same source, the problem seems to have been the practice of quota sampling:
By this time, all major polls were using what was belived to be a much more scientific method for choosing their samples called quota sampling. Quota sampling had been introduced by George Gallup as early as 1935 and had been successfully used by him to predict the winner of the 1936,1940 and 1944 elections. Quota sampling is nothing more than a systematic effort to force the sample to fit a certain national profile by using quotas: The sample should have so many women, so many men, so many blacks, so many whites, so many under 40, so many over 40 etc. The numbers in each category are taken to represent the same proportions in the sample as are in the electorate at large.
If we assume that every important characteristic of the population is taken into account when setting up the quotas, it is reasonable to expect that quota sampling will produce a good cross-section of the population and therefore lead to accurate predictions. For the 1948 election between Thomas Dewey and Harry Truman, Gallup conducted a poll with a sample size of about 3250.