Types of method

• Quantitative:– Questionnaires– Experimental designs

• Qualitative:– Interviews– Focus groups– Observation

• Triangulation

What is sampling? (Babbie)

• Polls and other forms of social research, rest on observations.

• The task of researchers is to select the key aspects to observe, or sampling.

• Generalizing from a sample to a larger population is called probability sampling and involves random selection.

Advantages of Sampling

• Less costs– cheaper than studying whole population

• Less errors due to less fatigue– better results

• Less time– quicker

• Destruction of elements avoided– eg bulbs

Concepts in sampling

• Population (or target population)– entire group of people, events or things of interest that the

researcher wishes to investigate • Element

– a single member of the population • Sampling Frame

– a listing of all the elements in the population from which the sample is drawn

• Sample– a subset of the population

• Subject– a single member of the sample

Sampling decisions1. What is the relevant target population of focus to

the study?2. What exactly are the parameters that we are

interested in studying?3. What kind of sampling frame is available?4. Should a probability or non-probability method be

chosen?5. What is the sample size needed?6. What costs are attached to the sampling?7. How much time can be spent collecting the data?

Probability & Non-probability Sampling

• Probability Sampling– Used when researchers want precise, statistical descriptions of

large populations.– In order to provide useful descriptions of the total population, a

sample of individuals from a population must contain the same variations that exist in the population.

– the elements in the population have some known chance or probability of being selected as sample subjects

• Non-probability Sampling– the elements do not have a known or predetermined chance of

being selected as subjects – Difficult to generalise to the population

• Decision tree - SLT ch.11

Types of Probability Sampling

• Simple random sampling– every element in the population has a known and

equal chance of being selected as a subject – Not the most accurate method

• Complex (or restricted) probability sampling– procedures to ensure practical viable alternatives

to simple random sampling, at lower costs, and greater statistical efficiency

Simple Random Sampling

• Is the most representative of the population for most purposes

• Disadvantages are:– Most cumbersome and tedious– The entire listing of elements in population

frequently unavailable– Very expensive– Not the most efficient design

Complex Probability Sampling

• Systematic sampling• Stratified random sampling• Cluster sampling• Area sampling

Multi-stage sampling

Non-probability Sampling

• Convenience sampling– Survey whoever is easily available– Used for quick diagnosis of situations

• Simplest and cheapest• Least reliable

• Purposive sampling– Judgement sampling:

• Aim is to achieve “saturation” of cases

– Snowball sampling– Quota sampling

Qualitative sample designs

• Not necessarily representative of population• Use non-probability methods• Key issue: avoiding systematic bias• Avoid ‘paralysis by analysis’