2.4 Bias in Surveys

Bias

• Bias occurs when a sample is not representative of the population due to an unintended (or intended) influence in the data gathering.• Data collected with bias distorts the truth• Students in MDM4U are surveyed to determin attitude of all students

in the school.

Sample and Population

• Population is every member of a certain group of people. • Sample is pick as a small representative part of the larger population.

Bias in the Sample: Sampling Bias

• Occurs when the sample does not accurately reflect the characteristics of the population.• A survey asked people at a rave whether taxes should subsidize fog

machines and red bull or education• Students at the hockey game are asked whether the school should

fund new equipment for the hockey team.

Non-Response Bias

• Occurs when particular groups are under represented in a survey because they choose not to participate.

“ A science class asks every fifth student entering the café to answer a survey on environmental issues. Less than half agree to take the survey. The completed survey shows a large amount of respondents are concerned about environment”

Bias in the Survey

Measurement Bias

• Occurs when collection method consistently either under- or overestimates a characteristic of the population.

• As part of a survey of the “Greatest country singers of all time” a radio asks which is the best:

a) Travis Tritt b) Dwight Yoakumc) Garth Brooks d) Other:__________

Measurement Bias Con’t

• A highway engineer suggest that to survey traffic speeds police officers could patrol the highway every half hour and record speed of traffic.

Respondent Bias

Response Bias

• Occurs when participants in a survey deliberately give false or misleading answer.• May be a result of poorly written questions, openly biased

interviewer, and questions that are sensitive.• A police officer approaches you on the street, “Hello, I am completing

a survey. Have you smoked pot in the last couple of hours?”

Response Bias Cont.

• Teacher asks students to raise their hand if they didn’t understand the homework• Given that the threat of nuclear war is high now that it has ever been

in human history, and the fact that a nuclear war poses a threat to the very existence of the human race, would you favor and all-out nuclear test ban?

What kind of Bias?

• You wish to find out how many hours teens spend playing video games on an average school night, so you survey your school’s computer lab.

• Sampling Bias: The likelihood that members of the computer club play more video games is quite high.

What kind of Bias?

• You wish to determine how many students will come to an upcoming dance and so you email 100 surveys to students in each grade.• Non-Response Bias: People might not respond unless they are

pumped about dance.• Sample Bias: is 100 for each grade representative of each grade? 600

grade 10’s and only 100 grade 12’s?

What Kind of Bias?

• To Collect data on teen shopping habits, you send surveys to every third house on your street.

• Non-Response bias: you not be targeting places where teens live so they won’t reply.• Sample bias: Homes may be in wealthy or poor neighborhood.

What Kind of Bias

• You take a random sample of females to survey them about students preference towards various potato chips.

• Sampling Bias: Both males and females should be surveyed, could skew results having only one gender.

What Kind of Bias?

• You survey the chess club to determine how many hours of television teenagers in your school watch per week.

• Sample Bias: Chess club members aren’t a fair representation of all teens in school.

Where is bias and how to fix?

Assignment

• Pg 123 #’s 1-6,8