Lesson Notes 6.6: Analyze Surveys and Samples

·  A survey is a study of one or more characteristics of a group.

·  The entire group you want information about is called a population.

·  In a random sample, every member of the population has an equal chance of being selected.

·  In a stratified random sample, the population is divided into distinct groups. Members are selected at random from each group.

·  In a systematic sample, a rule is used to select members of the population.

·  In a convenience sample, only members of the population who are easily accessible are selected.

·  In a self-selected sample, members of the population select themselves by volunteering.

·  A representative sample is a sample that accurately reflects the characteristics of a population.

·  A biased sample is a sample that is not representative of the population.

·  A question that encourages a particular response is a biased question.

Sampling Method / How Members are Chosen
Random / By Chance
Systematic / According to a rule or formula (every 10th one)
Stratified / At random from randomly chosen subgroups
Convenience / Easiest to reach
Self-selected / Members choose to be in the sample

Examples of sampling methods:

o  An exit poll taken of every tenth voter

Ø  Systematic – The rule is to question every tenth voter.

o  Listeners are invited to call in to a radio show to voice their opinions.

Ø  Self-selected – Callers choose to participate.

o  A telephone company randomly chooses customers to survey about its service.

Ø  Random – The participants are chosen by chance.

o  A high school randomly chooses three classes from each grade and then draws three random names from each class to poll about lunch menus.

Ø  Stratified – Students are chosen at random from the three classes that were selected at random.

o  The customers in a hair salon are surveyed.

Ø  Convenience – The customers were surveyed because they were in the salon.

Examples of Potentially Biased Samples

§  A teacher asks students who buy lunch if they like the cafeteria food. (If they eat the food in the cafeteria, they are more likely to be the ones who like cafeteria food.)

§  An architecture firm asks people attending a city council meeting which design for a new city hall they prefer. (Most citizens don’t attend city council meetings.)

§  A biologist studying trees samples blossoms of trees along the river. (The trees sampled have a better source of water than other trees.)

§  A pet store owner surveys 100 customers to find out which brand of dog food is most frequently purchased. (Not all customers have dogs.)

Examples of Potentially Biased Questions

§  Is your favorite subject math or something else?

§  Don’t you think choice A is better than choice B?

§  Because there are not enough entertainment outlets in the county, do you think the county should search for someone to open a movie theater?

§  Would you pay higher taxes to fund a new park?