Sampling technique  It is a procedure where we select a group of subjects (a sample) for study from a larger group (a population)

Why do sampling  A large proportion of individuals or items or units have to be studied, we take a sample  Easier and economical to study sample than the whole population  Items in the sample are representative of the whole population to be studied

Target population The population to which the investigator wishes to generalise

 The population of sampling units from which we draw our sample.  For practical reasons, the study population is often more limited than the target population Study population (Survey population)

Sample  A subset of the population, selected so as to be representative of the larger population

Sampling  Sampling frame: The set of sampling units from which a sample is to be selected. E.g. a list of names, or places etc.

Sampling  Sampling unit: The unit of selection in the sampling process. E.g. a person, a household or a district. It is not necessarily the unit of observations or study

Sampling  Sampling error A difference that occurs purely by chance between the value of a sample statistic and that of the corresponding population parameter

Types  A probability sampling : in which every unit in the population has the same chance or probability of selection.  Non probability sampling : is any sampling method where some elements of the population have no chance of selection

Factors commonly influencing the choice between these types include:  Nature and quality of the frame  Availability of auxiliary information about units on the frame  Accuracy requirements, and the need to measure accuracy  Whether detailed analysis of the sample is expected  Cost/operational concerns

Probability Sampling

Simple random sampling  Each element of the frame thus has an equal probability of selection  Minimizes bias and simplifies analysis of result  SRS can be vulnerable to sampling errors may result in a sample that doesn't reflect the makeup of the population(male : female)

 Advantages: 1. Easy to estimate the accuracy of results 2. Easy &simply  Disadvantages: 1. A sample that doesn't reflect the makeup of the population. 2. It does not provide subsamples of the population.

Systematic sampling  relies on arranging the target population on ordered list and then selecting elements at regular intervals.  Advantages: 1. It is easy to implement. 2. the stratification can make it efficient.

 Disadvantages: 1. The sample likely to be unrepresentative, making the study less accurate 2. Difficult to quantify that accuracy.

Stratified sampling  The population frame can be organized by categories into separate strata. Then individual elements can be randomly selected.  Potential benefits: 1. Enable researchers to draw inferences about specific subgroups. 2. Can lead to more efficient statistical estimates. 3. Enabling researchers to use the data collection approach best suited for each subgroup.

 Potential drawbacks: 1. Increase the cost and complexity of sample selection. 2. Can potentially require a larger sample than would other methods.

 A stratified sampling approach is most effective when: 1. Variability within strata are minimal. 2. Variability between strata are maximum.

Cluster sampling  To select respondents in groups ('clusters'). Sampling is clustered by geography, or by time periods.  It requires a larger sample than SRS to achieve the same level of accuracy.  It is commonly implemented as multistage sampling  Advantages: 1. Can reduce travel and administrative costs. 2. Does not need a sampling frame.

Non Probability Sampling

Convenience sampling  This sample would not be representative enough.  This type of sampling is most useful for pilot testing.

Quota sampling  One of non-probability sampling.  The population is first segmented into mutually exclusive sub-groups.  Then selection of the sample ( which is non- random) based on a specified proportion.  Disadvantage: These samples may be biased.

Panel sampling  Is the method of first selecting a group of participants through a random sampling method, then each participant is given the same survey or interview at two or more time points.