Bias!!

Journal Topic Why do we take Samples? Fill in the blank: A good sample is of a population. Define Bias We take samples in order to reduce the chance of bias within our results. representative

Task Pick 5 students in the class to represent the class. You will have 30 seconds… GO!

Bias Defined… Systematic Error due to a non-random sample of a population Refers to tendency of a sample statistic to systematically over- or under- estimate a population. Feelings and opinions not facts Often occurs when the survey sample does not accurately represent the population.

Where can bias occur? Bias is most likely to occur within a study when the population is underrepresented. (Selection Bias) Undercoverage occurs when some members of the population aren’t represented in the sample Nonresponse bias individuals chosen for the sample are unwilling or unable to participate in the survey Voluntary response bias The resulting sample tends to overrepresent individuals who have strong opinions.

What is the best sampling technique to reduce bias? Simple Random Sample

Why? Selection of individuals is based on chance Every individual within the population has an equal opportunity to be picked.

Bias occurring in Surveys Response bias Refers to the bias that results from problems in the measurement process. Leading questions Questions tend to lead the individual to a certain response. Social desirability Most people like to present themselves in a favorable light, so they will be reluctant to admit to illegal activities in a survey, particularly if survey results are not confidential. Instead, their responses may be biased toward what they believe is socially cool.

Write your response on a notecard and turn it in Which of the following statements are true? I. Random sampling is a good way to reduce response bias. II. To guard against bias from undercoverage, use a convenience sample. III. Increasing the sample size tends to reduce survey bias. IV. To guard against nonresponse bias, use a mail-in survey.