Producing DataLab #3

Reading correlation table: Bottom of p. 52 and top of page 53 of Sorenson

Reading regression output to construct your equation: Sorenson p. 55. You need information in “coefficients” table – table 5.2

Producing DataSAMPLING

Exploratory Data Analysis

the use of tables and graphs to aid in describing of quantitative data

Conclusions pertain to specific data

What happens when we wish to answer a question about large groups of individuals

We must collect data in such a way that will allow answering such questions

E.g. What are the attitudes of American population towards gambling?

Census

Samples

Producing DataTwo ways of gathering sample data

Observational Study

Observes individuals and measures variables without the attempt to influence the responses.

Ask students how they feel about cumulative final exams – sample survey/opinions

A biologist studies the behavior of lobsters through observation of such behavior

Producing DataTwo ways of gathering sample data

Experiment

Imposes some treatment on individuals in order to observe response

To understand cause and effect

Involves intervening not just observingMedical experiment where 20,000 male physicians over age 40 were used to test the theory that aspirin reduces risk of heart attack

Producing DataConfounding

Observational studies should not be used in the study of effect of one variable on another

Explanatory variable is confounded with lurking variables.

Two variables are confounded when their effects on a response variable cannot be distinguished from each other

Moderate use of alcohol is associated with better health. Some studies suggest that drinking wine rather than beer or spirits confers added health benefits. Confounding?

Producing DataBut, people who prefer wine are different from

those who drink mainly beer and stronger stuff. Wine drinkers eat more fruit and vegetables and less fried foods. Their diets contain less fat, less cholesterol, and also less alcohol. They are less likely to smoke. Finally, wine drinkers as a group are richer and better educated than people who prefer beer or spirits.

Conclusion from study: The apparent health benefits of wine compared with other alcoholic beverages, as described by other, may be a result of confounding by dietary

habits and other lifestyle factors.

Designing Samples

Sample Design – method used to choose the sample from the population

Sampling design practices are extremely important as improper designs may render experimental results to be misleading or invalid

TYPES OF SAMPLING

NON-PROBABILITY SAMPLES

VOLUNTEER SAMPLING

Sample based on individuals who volunteered to participate in a study – choose themselves for study

CONVENIENCE SAMPLING

Choosing individuals for study based on availabilityIndividuals are easiest to reach

Corner interviews; call-in telephone polls; mall surveys, etc..

Samples open to bias – sampling bias – systematically favors choosing certain individuals for samples, and thus favors certain outcomes or conclusions from the data

TYPES OF SAMPLING

PROBABILITY SAMPLES

Attempt to minimize bias

1. SIMPLE RANDOM SAMPLE (SRS)

Consist of individuals from a population chosen in such a way that every set of n individuals has an equal chance to be the sample actually selected

Srs gives each individual an equal chance of being chosenSrs gives each sample an equal opportunity of being chosen

SAMPLING FROM A FINITE POPULATION

TYPES OF SAMPLING

PROBABILITY SAMPLES – samples chosen by chance

1. SIMPLE RANDOM SAMPLE (SRS)

IDEA: to choose our sample by drawing names from a hat.Let computer software randomize via TABLE OF RANDOM DIGITS

A table of random digits is a long string of the digits 0 through 9 with these properties:

1. Each entry in the table is equally likely to be any of the 10 digits

• No value produced by a random number generator should be any more likely than any other

• Probability distribution of numbers produced must be uniform

2. The entries are independent of each other.• There should be no possibility of predicting

the next number before it appears

• In random samples, each individual has an equal chance of being chosen into the sample

TYPES OF SAMPLING

Chose a simple random sample of 5 individuals from the following list of students

Adam Trisha Donna Paul John Oscar George Maria Anna Catherine Michael Robert