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Will consider three aspects of reliability:

1.Sources of unreliability.

2. Testing reliability.

e.g. test re-test method.

3. Increasing reliability



Three basic ways to access validity:

1. Criterion validity.

2. Content validity

3. Construct validity.


Types of probability sample

SRS -Simple random sampling

5 steps:

  1. Obtain a complete sampling frame.
  2. Give each cause a unique number starting at one.
  3. Decide on the required sample size.
  4. Select that many numbers from a table of random numbers.
  5. Select the cases which correspond to the randomly chosen numbers.

Systematic sampling.

Divide population by required sample size e.g. 50/10 = 1/5. We will select one person for every 5 in the population. Where to start? Since fraction = 1/5 - starting point is somewhere between persons 1 - 5 , so decide from table of random numbers.

Stratified sampling.

Multistage cluster sampling.


Non-probability sampling.


1. Purposive sampling.

2. Quota sampling.

3. Availability sample.


Sample size

The required sample size depends on two key factors:

Need to decide how much error we are prepared to tolerate.