Stat 20: Intro to Probability and Statistics Lecture 2: Surveys and Sampling

Tessa L. Childers-Day UC Berkeley

24 June 2014

Today’s Goals

Survey Basics and Language

Types of Surveys

Examples

By the end of this lecture...

You will be able to: Recognize the similarities and differences between surveys and observational studies Use statistical language to describe the elements of a survey Apply your knowledge to assess the strengths and weaknesses of a given survey design

2 / 14

Today’s Goals

Survey Basics and Language

Types of Surveys

Examples

Studies and Surveys Differences Observational Study: attempts to isolate the effect of a change/manipulation–but investigator does not control the manipulation, only observes it Sample Survey: Collects information about a particular topic from a sample

Similarities Generalize (infer) from small group observed (sample) to larger unobserved group (population) Validity of conclusions depends on how good/representative sample is Bias is a concern for both (non-adherence or non-response, experimenter or interviewer selection bias)

3 / 14

Today’s Goals

Survey Basics and Language

Types of Surveys

Examples

Survey Vs. Census

Why make generalization from sample to population? Why don’t we just look at the population directly (census)? Cost Accuracy Destruction

4 / 14

Today’s Goals

Survey Basics and Language

Types of Surveys

Examples

Survey Jargon Recall that a survey collects information about a particular topic from a sample, in order to make inferences about the population Example: What’s the average student loan debt of a Cal student? Population: All Cal Students Sample: Cal Students in this Stat 20 class Parameter: Average debt of population Statistic: Average debt of sample

5 / 14

Today’s Goals

Survey Basics and Language

Types of Surveys

Examples

Survey Jargon (cont.)

Is it a representative sample? Is there selection bias?

Are there a large number of non-responses?

Are the questions worded neutrally? Is the interviewer neutral?

6 / 14

Today’s Goals

Survey Basics and Language

Types of Surveys

Examples

Non-Random Samples

These samples are never random, chance is totally un-planned, un-quantifiable Quota Sample

Convenience Sample

Purposive Sample

7 / 14

Today’s Goals

Survey Basics and Language

Types of Surveys

Examples

Possibly Random Samples

These samples might be random, chance might be planned, quantifiable Cluster Sample

Stratified Sample

Systematic Sample

8 / 14

Today’s Goals

Survey Basics and Language

Types of Surveys

Examples

Definitely Random Samples

This sample is always random, chance is totally planned and quantifiable Simple Random Sample

All samples that involve quantifiable chance are probability samples

9 / 14

Today’s Goals

Survey Basics and Language

Types of Surveys

Examples

Combining Sample Types

Often use multistage sampling to combine sampling types (usually random): Multistage Random Cluster Sampling

10 / 14

Today’s Goals

Survey Basics and Language

Types of Surveys

Examples

Example 1: Student Loans

Interested in measuring average student loan debt. Give each of you a written survey: Student loan debt is rising across the country, and more and more students are having trouble finding jobs that pay enough to repay their student loan debts, sending them into default. How much debt do you have? Please specify Do you think it is too much? Yes/No Do you think you will go into default? Yes/No

11 / 14

Today’s Goals

Survey Basics and Language

Types of Surveys

Examples

Example 2: Oral Hygiene Interested in finding out how people take care of their teeth. Send a dental hygienist in scrubs to ask questions of randomly selected patients from a pediatric dentist, a general dentist, and an oral surgeon. The mouth is a breeding ground for bacteria and other germs, that if not properly cared for can result in pain, infection, tooth loss, and even heart problems. With the wide availability of tooth brushes and dental floss, proper dental hygiene is accessible to all people. Do you brush your teeth twice daily as recommended? Yes/No Do you floss twice daily as recommended? Yes/No Do you visit the dentist twice yearly as recommended? Yes/No 12 / 14

Today’s Goals

Survey Basics and Language

Types of Surveys

Examples

Example 3: Sex Habits on Campus

Interested in sexual habits of students. Who should we ask? Where/how will we find them? What questions should we ask? How (or by whom) should response be gathered?

13 / 14

Today’s Goals

Survey Basics and Language

Types of Surveys

Examples

Important Takeaways

Surveys use sample statistics to estimate population parameters The best samples feature Impartiality/no discretion on the part of the interviewer The planned (quantifiable) use of chance/randomness

Main concern: minimize/eliminate bias Selection bias Non-response bias Response bias

Next time: Types of Data and Displays

14 / 14