1.  Systematic
 Convenience
 Purposive
 Population
Genaralizability
 Ecological
Genearalizabilit
y

2.  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.

3.  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.

4. 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.

5.  Bias-
 1. not downtown = not interviewed
 2. unwilling = not interviewed
 3. willing = strong opinion
 4. interview time = at work
 Ingeneral, convenience sample cannot be
considered representative of any population
and should be avoided.

6.  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.

7.  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

8.  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.

9.  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.