Data Gathering Techniques Samples and Surveys

2 Types of Data 1. Numerical 2. Categorical

Numerical examples include things like how many, height, weight, salary, etc

Categorical examples include things like what kind, what type, what color, etc ask for specific examples, ie sports, movies, eyes, hair...

Population all members of a set

Sample Part of a Population If you determine a sample carefully, the statistics for the sample can be used to make general conclusions about the larger population.

Sampling Types Methods used to select samples Convenience Self-selected Systematic Random

Convenience Sample select any members of the population who are conveniently and readily available define different sample types by the methods used to select them

Self-selected sample select only members of the population who volunteer for the sample

Systematic sample order the population in some way, like alphabetically, and then select from it at regular intervals

Random sample all members of the population are equally likely to be chosen

Bias A systematic error introduced by the sampling method. A sample has a bias when a part of a population is overrepresented or underrepresented

Identify the sampling method. Determine if there is any bias. A newspaper wants to find out what percent of the city population favors a property tax to raise money for local parks. A newspaper article on the tax increase invites readers to express their opinions on the newspaper’s website. self-selected; bias depends on who visits website

Identify the sampling method. Determine if there is any bias. A newspaper wants to find out what percent of the city population favors a property tax to raise money for local parks. A reporter interviews people leaving the city’s largest park. convenience; bias overrepresents park supporters

Identify the sampling method. Determine if there is any bias. A newspaper wants to find out what percent of the city population favors a property tax to raise money for local parks. A survey service calls every 50th listing from the local phone book. systmatic, bias if there is some link btw people listed & people who pay property tax

Identify the sampling method. Determine if there is any bias. The students planning the junior class party want to know what kinds of pizza to buy. They ask the pizza restaurant what kinds sell the most. convenience, restaurant sells to other people besides students (underrep.)

Study Methods another way to collect sample information is to perform a study observational controlled survey

Observational study measure or observe members of a sample in such a way that they are not affected by the study ask for examples of things that are studied, ie. student behavior, shoppers, etc

Controlled experiment Divide the sample into 2 groups. Impose treatment on one group but not on the other “control” group Then compare the effect on the treated group to the control group.

Survey Ask every member of the sample a set of questions

analyzing study methods Which type of study method is described in each situation? Researchers randomly choose two groups from 10 volunteers. Over a period of 8 weeks, one group eats ice cream before going to sleep, and the other does not. Volunteers wear monitoring devices while sleeping, and researchers record dream activity. Should the sample statistics be used to make a general conclusion about the population? controlled experiment. stats are based on small sample, findings unreliable for general conclusion

analyzing study methods Which type of study method is described in each situation? Students in a science class record the height of bean plants as they grow. Should the sample statistics be used to make a general conclusion about the population? observational study. may make conclusions, however soil type, amt of sunlight, water, fertilizer and other factors my affect the growth

analyzing study methods Which type of study method is described in each situation? Student council members ask every 10th student in the lunch line if they like the cafeteria food. Should the sample statistics be used to make a general conclusion about the population? survey. (convenient systematic) results not reliable b/c people in like are more likely to enjoy cafe food than those who brought lunch from home. WHAT TYPE OF SURVEY IS MORE LIKELY TO YIELD BETTER RESULTS?

analyzing study methods Which type of study method is described in each situation? A pharmaceutical company asks for volunteers to test a new drug to treat high blood pressure. Half the volunteers will be given the drug, and half will be given a placebo. The researcher will monitor the blood pressure of each volunteer. Should the sample statistics be used to make a general conclusion about the effectiveness of the drug in the larger population? controlled study, reliable, if other factors are random like age, gender, and overall health

Designing a survey SPORTS During the 2008 Olympic Games, a U.S. swimmer won more medals than any other swimmer in history. What sampling method could you use to find the percent of students in your school who recognize that swimmer from a photograph. systematic, every nth entering school (usually least amt of bias)

survey question SPORTS During the 2008 Olympic Games, a U.S. swimmer won more medals than any other swimmer in history. What is an example of a survey question that is likely to yield information that has no bias? “Who is pictured in this photograph?” if photo does NOT contain things like swimming pool or medals or Olympic symbol

ADDITIONAL PROBLEMS GIVE TYPE OF SAMPLING METHOD DOES THE SAMPLE HAVE A BIAS?

Politics A political candidate wants to know what percent of his constituency favors a Referendum A on an upcoming ballot election. His staff asks each person who comes into the candidate’s office for 3 days whether they support Referendum A. convenience, may share candidate’s opinions more than population as a whole

Management The manager of a grocery store wants to determine what percent of shoppers use store coupons. He asks every 10th shopper who passes through the store’s door for the next week if he or she intends to use a store coupon on his or her visit. systematic, no bais

study method? use stats? Scientists study the effects of the phases of the moon on the levels of low tide and high tide in 200 coastal cities in the U.S. for a period of 12 months. observation, reliable since several locations used and data collected over long period of time

study method? use stats? A pet food company randomly chooses two groups of 20 puppies for a study. One group eats a new brand of dog food and the other group eats an off-brand of dog food for a period of 4 weeks. Researchers record muscular and skeletal growth rates in both groups. controlled, unreliable b/c small sample and type of puppies

study method? use stats? For a class project, students in Health class ask every tenth student entering the school if they eat breakfast in the morning. survey, systematic. reliable b/c random students selected

NCFE type question To determine the most popular brands of tea consumed by Americans, a survey is conducted in a busy downtown location at lunch time. Which of the following is NOT a potential bias in the sampling method?

least biased Urban office employees are not representative of the general population. The results could be influenced by national brand teas available in the area. A lunchtime survey does not reflect peoples’ tastes at other times of the day. The survey must include call-in and online responses. d

Take note of samples and surveys used in different places. math is everywhere