Types of Samples and Observation, Populations vs. Samples, Types of Sampling, Errors in Sampling, Errors of Observation, Conclusion.

1. Samples and
Observation
Dadabhoy Institute of Higher Education
January 17, 2015

2. Populations vs. Samples
Who = Population:
all individuals of interest
US Voters, Dentists, College students, Children
What = Parameter
Characteristic of population
Problem: can’t study/survey whole pop
Solution: Use a sample for the “who”
subset, selected from population
calculate a statistic for the “what”

3. Representative Sample
 Sample should be representative
of the target population
 so you can generalize to population
 Random sampling
 All members of pop have equal chance of being
selected
 Roll dice, flip coin, draw from hat

4. Types of Sampling
 Simple Random Sample
 Stratified Random Sample
 Cluster sampling
 Systematic
 Convenience

5. Simple Random Sample
Every subset of a specified size n from the
population has an equal chance of being selected

6. Stratified Random Sample
The population is divided into two or more groups
called strata, according to some criterion, such as
geographic location, grade level, age, or income,
and subsamples are randomly selected from each
strata.

7. Cluster Sample
The population is divided into subgroups (clusters)
like families. A simple random sample is taken of
the subgroups and then all members of the cluster
selected are surveyed.

8. Systematic Sample
Every kth member ( for example: every 10th
person) is selected from a list of all population
members.

9. Convenience Sample
Selection of whichever individuals are easiest to
reach
It is done at the “convenience” of the researcher

10. Errors in Sampling
Non-Observation Errors
Sampling error: naturally occurs
Coverage error: people sampled do not match the
population of interest
Underrepresentation
Non-response: won’t or can’t participate

11. Errors of Observation
Interview error- interaction between interviewer
and person being surveyed
Respondent error: respondents have difficult time
answering the question
Measurement error: inaccurate responses when
person doesn’t understand question or poorly
worded question
Errors in data collection