Systematic

Convenience

Purposive

Population Genaralizability

Ecological Genearalizability

Every nth individual in the population list is

selected.

eg: the principal of a school has 1000

students, she wants to know how students feel

about the new menu at cafeteria, so the

principal:-

1. get a list of the student’s name (in

alphabetical order).

2. she select every 10th student until she has a

sample of 100 students to be interviewed.

PERIODICITY-a marked bias sample caused by

the arrangement pattern of individual on the

list accidentally coincides with the sampling

interval.

eg: grouped by gpa(grade point average),

high/low interval: only good/poor graded

get chosen.

Therefore, researchers should carefully

examine the list and avoid bias.

CONVENIENCE SAMPLING

A convenience sample is a group of

individuals who conveniently available.

samples:

1. first 50 people who walk in.

2. interview people at downtown.

3. two front rows students.

Bias-

1. not downtown = not interviewed

2. unwilling = not interviewed

3. willing = strong opinion

4. interview time = at work

In general, convenience sample cannot be

considered representative of any population

and should be avoided.

PURPOSIVE SAMPLING

Based on previous knowledge of a population and

the specific purpose of the research,researcher

use personal judgement to select a sample.

eg:SUITABLE SAMPLE

a. 2 good students,2 average & 2 weak

b. sample from Retired Workers Association

eg: sample know the target

A. people in charge of school

B. people with experience

THUS, the only challenge with purposive sampling is the

researcher’s previous knowledge must be thorough.

Sample should be as large as a researcher

can obtain with a reasonable expenditure of

time and energy.

Ideals;

100 samples for DESCRIPTIVE STUDY

50 samples for CORRELATION STUDY

30 samples in each group for EXPERIMENTAL

STUDY& CAUSAL-COMPARATIVE STUDY

External Validity = the result of a study can be

generalised from sample to population.

Ecological generalizability = result of a study can

be generalised to other settings.

Population generalizability = result of study can

be generalised to the intended population

-representativeness: relevant characteristics

Overlooked “method”-random student=random

teacher=random result.

Lost subject effect representativeness, researcher

who lost 10 % sample are advised to

acknowledge this limitation.

1. researcher should describe the sample

thoroughly; reader judge the result validity.

2. Replication; repeat the study on different

group or situation. If result is the same;

generalise it.