I am conducting a study to assess the quality of selected parts of some herbal materials and also develop acceptance criteria for their quality attributes. I am supposed to sample these materials from across the length and breadth of the country and I am hoping to stratify the country into strata and further divide each stratum into clusters, and then randomly sample the materials from each of the clusters picked up through systematic sampling.
My challenge is with the calculation of a 'realistic' sample size that can then be used to determine the number of clusters and the number of samples from each cluster. Very often what I see in literature tends to be convenient sampling, which may not be representative of the population. The focus of my study however requires that my sampling is representative of the population in the country (and also realistic), especially because of the part that has to do with setting acceptance criteria.
I would be very grateful for your technical assistance. Thank you.
Hi Emmanuel,
I think what you have not said about your project is also important. When you say quality how do you measure it? If quality attributes is of morphological, it is relatively easier and can be assessed along with your acceptance criteria. If your quality measure is on chemical profiling, however, then you need to define your clusters and representative sampling volume according to the potential causes of variation on the chemical composition of the plants in question.
You did not mention what plants you are dealing with – cultivated or wild – as this has implication on chemical variability. If wild, parameters of chemical variation would include, climatic differences, altitudes, soil type, latitude (if it is a big country), etc. For cultivated ones, you may add farming methods and other human input in addition to the above-mentioned potential variations. Your cluster size depends on the degree of variations you have in the country while sampling size depend on what assessment you use to present your data through appropriate statistical analysis.
Have fun 😊
Thank you for the valuable inputs Solomon Habtemariam . Some of the variables you indicate have actually been considered: Both morphological and chemical profiling (including content assays and impurity profiling) would be considered. While most of these materials are usually collected from the wild, I am hoping to sample some cultivated ones too (hence, your suggestions are valuable).
If it is possible, I would love to contact you for further clarification. Thank you once again.
Hi Emmanuel,
A country is too large for this type of survey. I guess you need to use multi-stage sampling also known as cluster sampling.
You will need to stratify the country into separate homogenous units (e.g. based on ecology, ethnicity, administrative units, etc.). Within each cluster again find a homogenous distinguishing feature. Go on identifying subunits until the smallest possible one and then randomly selected units within the last unit. A very useful text on this sampling approach is by De Vaus (2014). De Vaus, D. 2014. Surveys In Social Research. 6th edition. Routledge.
What I have not yet figured out is if I have e.g. 10 units at the second stage (cluster), what proportion I should take of these 10.
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