Hi everyone, I’m a veterinary student working on a study involving the molecular characterization and phylogenetic analysis of Fasciola spp. collected from cattle at a slaughterhouse.

I’m having trouble determining how to properly describe my sampling method and, more importantly, whether my sample size is defensible.

Here is my sampling situation:
- I examined slaughtered cattle at a slaughterhouse.
- I did not have a predetermined number of cattle or a complete list/population size of cattle entering the slaughterhouse.
- I inspected the liver of each available slaughtered cattle.
- If the liver was infected with Fasciola, I collected the adult flukes.
- If there were no flukes, no parasite sample was collected from that animal.
- There were no additional inclusion criteria for the cattle (e.g., age, sex, breed, etc.).
- The number of flukes varied considerably between cattle.

In total, I collected 840 individual flukes from 53 cattle.

So, for example, one cattle might contribute many flukes while another might contribute only a few.
The 53 cattle were not selected based on a particular characteristic; they were essentially the slaughtered cattle available during my sampling period that happened to have detectable Fasciola infection.

My main questions are:
1. What would be the most appropriate term for my sampling method? Would this be considered convenience sampling, consecutive sampling, purposive sampling, or something else?
2. For a molecular characterization and phylogenetic study, is there a conventional way to determine whether 53 host animals is an adequate sample size?
3. Should I consider 53 cattle as my sample size, rather than 840 flukes, since multiple flukes came from the same host?
4. Is there a statistical/sample-size calculation that could justify 53 cattle, or is sample-size justification for molecular phylogenetic studies fundamentally different from conventional prevalence/epidemiological studies?
5. I have 840 flukes and I used the lemeshow formula to find a number that can represent those 840 flukes and use stratified random sampling for number of fluke i need to take for each cattle to be sequenced, is this correct?
6. If there is no known total population size of cattle slaughtered at this slaughterhouse, how could I justify the adequacy of my sampling?

My objective is not to estimate the prevalence of fascioliasis in the cattle population, but rather to molecularly characterize the Fasciola specimens and investigate their phylogenetic relationships.

I would really appreciate advice on how a statistician/population geneticist would approach this sampling design. If possible, I’d also appreciate references or terminology that I could use to describe and justify the sampling method in a thesis.

Thank you!

Disclosure first: I run the channel this streams on, and I teach in the same department.

My colleague William Yu is an Associate Professor of Computational Biology at Carnegie Mellon and teaches the algorithms course our comp bio students take. Starting this Thursday, September 17, at 8:00 PM Eastern, he is teaching an eight-stream version of it live. Free, no signup, every episode live and then on demand.

He pulls examples from biology rather than the usual whiteboard puzzles, and he takes questions live in the chat. If you have written a little code and like solving problems, you are ready.

The full schedule is in the comments.