But clinical trials are difficult to perform with children.
However, dry friction is highly non-linear, and hence, analytical investigations are difficult to perform.
Steps 2 and 4 are difficult to perform in rodents without disturbing the animal.
The current methods to identify rRNA modifications are difficult to perform and need expensive reagents.
Such manipulations are difficult to perform with precision, because the bonds between the carbon and hydrogen atoms are strong.
Questionnaires measuring health-related quality of life are difficult to perform and obtain for patients and professionals.
Before you throw out your biology textbooks, however, Ferguson-Smith warns that transgenerational epigenetic experiments are difficult to perform and can be misinterpreted.
Current migration assays for screening small molecules, siRNAs, or other perturbations are difficult to perform in parallel at the scale required to screen large libraries.
One challenge is that longitudinal diet studies are difficult to perform because there are so many variables in what people eat over long periods of time.
Direct tensile test are difficult to perform and actually no consensus on phenomenon involve and on tensile creep compliance (with respect to compressive one) is available.
These projects required continuous costs and are difficult to perform in developing countries.
