Types of Bias How to pick the right sample

What is bias? Bias is any inconsistencies in using a sample to make inferences about the entire population. This can be our sample not being representative of the population, the answers not being valid, or people in your sample not cooperating with your work. Our goal is to try and minimize bias when we collect our sample.

Types of bias Undercoverage Response bias Nonresponse bias Volunteer bias Convenience sampling

Undercoverage Undercoverage is when a certain part of the population is not able to be included in the sample, whether it be by the sampling method or method of collection. This leads to your sample not being representative of the population, which voids the validity of your results.

Undercoverage II A good example of undercoverage is phone polling. If you want to include all adults in the US in your sample, you are leaving out all people who do not own phones or have landlines. Since they are not even able to be included in the sample, our sample cannot be representative of the entire US population.

Response bias Response bias is when people respond differently than they truly feel. This is common when the questions being asked are worded to make you feel a certain way. It also happens when the questions are about an emotional topic.

Response bias II When an environmentalist group asks questions about global warming, people might respond differently because they know the position the group holds. The identity of the questioner should be hidden and the wording of the questions should be neutral to reduce this type of bias.

Nonresponse bias Nonresponse bias is not the opposite of response bias. Nonresponse bias occurs when people who are selected for the sample choose not to participate in the survey. This leads to a possibly less representative sample of the population.

Nonresponse bias II This also occurs when doing phone surveys. If you have a phone number selected to be in the survey, but no one ever answers, then that is a nonresponse bias. The difference is that response bias is a choice you make, but nonresponse is just not being included at all.

Volunteer bias Volunteer bias occurs by how you collect your sample. Instead of picking people beforehand, you simply ask people if they would like to be included in a survey. This bias leads to people who are outgoing or have strong opinions to be included in your sample. It also means the sample is not as representative.

Convenience Sampling Convenience sampling is just using people around you for your sample, instead of using some sort of randomness. This has the same problems as undercoverage and volunteer bias, where a lot of people will not be able to be included in the study, and only people who agree to help you are in your sample.

How to reduce bias? Good ways to reduce bias in your sample are to 1) take a large sample. 2) use random sampling methods, as opposed to convenience sampling. 3) do your best to make sure that all possible units can be included in your sample.

Examples Below the powerpoint are some examples to see how well you can identify which types of bias can be present in different situations.