Reliability

Will consider three aspects of reliability:

1.Sources of unreliability.

- may be due to bad wording
- a person may understand the question differently on different occasion.
- different coders might code the same responses (e.g. occupation) differently.

2. Testing reliability.

e.g. test re-test method.

3. Increasing reliability

- the best way to create reliable indicators is to use multiple item indicators: they are more reliable.
- asking their sex in different way.

Validity

Three basic ways to access validity:

1. Criterion validity.

- compare new question (our own) with the established measure - but the old one is valid/hard to find?

2. Content validity

3. Construct validity.

- evaluate a measures by how well the measure conforms with theoretical expectations.

Types of probability sample

SRS -Simple random sampling

5 steps:

- Obtain a complete sampling frame.
- Give each cause a unique number starting at one.
- Decide on the required sample size.
- Select that many numbers from a table of random numbers.
- 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.

- Select the relevant stratifying variable(s).
- Use SRS or systematic sampling to select of people within each strata.

Multistage cluster sampling.

Non-probability sampling.

1. Purposive sampling.

- by judged as typical of some category of cases of interest to the researcher.

2. Quota sampling.

- the quotas are given; e.g. 20% of population between 20 - 30 years old etc.
- non - random - select by a certain criteria.

3. Availability sample.

- anyone be useful for pilot testing questionnaire or exploratory research.
- must be used with caution and only for specific purposes.

Sample size

The required sample size depends on two key factors:

- the degree of accuracy
- the extent to which there is variation in the population.

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