It is impossible to sample and identify every species, even in small areas, and this is especially true for the hyperdiverse invertebrates [2].
Delimiting this range of conditions is a methodological challenge, because it is impossible to sample all potential field locations for any species in a given region.
Since it is impossible to sample the artery directly vascularizing the tumour, the gold standard is serial arterial sampling of a superficial artery (e.g. the radial artery) after which the activity concentration of the plasma derived by centrifugation can be determined [ 105].
Direct methods include coring, well logging, and well tests; In practice, coring is the most reliable method, but it is expensive and time consuming; Moreover, it may be impossible to take samples in some intervals of a well.
Above all, if the effect is small, it may be impossible to demonstrate without big sample sizes and multiple year studies.
"Occasionally the insertion of copper pipe can produce particularly bad results and despite satisfactory pH control it may be impossible to obtain any satisfactory samples," they wrote.
This difference in performance of the test with patient age may be explained by its apparent dependence on an overnight, fasting sample; such a sample would be impossible to obtain in children who are not toilet trained.
If exposure measurements are highly correlated, it may be impossible to distinguish hypotheses without a large sample size or many repeated measures.
Meat that had been further processed (e.g. sausages) was generally avoided, as it would be impossible to confirm the species of such samples through visual inspection.
Selling everything you could possibly want to consume – honey, coffee, sushi, cheese, bread, fruit, vegetables, cured meats, etc – and with a focus on locally sourced produce, it is impossible to resist sampling something.
