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