National EMSC Data Analysis Resource Center
It is common to read magazine polls that go something like this:
“In our last issue, we asked readers to respond to the following question: ‘Do you think your mechanic is honest?’ Based on this poll of our readers, 79% of Americans do not trust their car mechanic.”
There are a couple of reasons why polls of this sort are NOT RELIABLE:
Readers of “Fish and Game” or “Science" are not representative of the entire U.S. population. They tend to have proportionally more male readers and more readers that live in rural areas. Think of readers of “Fish and Game” or “Science.” Do you think they have different proportions of male and female readers than the United States population? Do you think they are more or less likely to live in urban or rural areas than the proportions in the United States population?
These are very specific groups of people that have characteristics that may not be representative of the United States population as a whole. Therefore, the sample results are biased and not representative of the population the magazine is making a conclusion about (Americans).
Randomly selecting the members of a sample is important because it helps prevent bias in your results. Random selection allows impersonal choice to choose the sample, rather than the individual performing the poll (the sampler) to select their own participants or self-selection of respondents as in the voluntary response poll mentioned above. If a sampler does not use random selection, the sample will favor selection of certain groups, possibly without the sampler even realizing it.
In the real world, a truly random sample is difficult to achieve, but you can come close. One of the most difficult steps is obtaining a complete list of every member in the population you want to sample from.
Many times, the telephone book for a city is used. Some of the problems associated with using the telephone book are it excludes those who do not have a telephone, those who have unlisted numbers, and more recently relevant, those who use a cell phone instead of a land line for all their calls.
A second barrier to purely random samples is for some people in the population, you will find it difficult or impossible to locate them. For example, people who work unusual hours or who travel a lot may be selected to be included in the sample, but are not available when you attempt to contact them.