True about cluster sampling is all except:
|A||Sample size same as simple random sampling|
|C||Independent of result|
|D||Done for evaluating immunization status|
Ref: Basic & Clinical Biostatistics; Dawson – Saunders & Trapp
- It is used when units of population are natural groups or clusters such as villages, wards, blocks, slums, and school.
- Cluster samples may be used when it is too expensive or laborious to draw a simple or stratified random sample; eg – in a survey in medical students in United states, an investigator may start by selecting a random set of groups or clusters – such as a random set of 10 medical schools in the USA.
- In cluster sampling the clusters are selected randomly, and all members in the cluster are sampled.
- This method is much more economical and practical than trying to take a random sample directly from the widely scattered population of all medical students in the United states.
- Cluster sampling gives a higher standard error but the data collection in this method is simpler and less expensive.
- Another disadvantage of this method of sampling is that inter-cluster comparison is not possible.
- Design effect is a phenomenon seen with cluster sampling and is the reason why sample size in cluster sampling is larger than that in simple random sampling.
- As per module approved by WHO, it is most often used to evaluate vaccination coverage in expanded programme of immunization or to find out missed cases in pulse polio immunization, a 30 / 7 (30 clusters with 7 subjects in each) cluster technique is used.