Hard evidence on this is difficult to collect.
It is difficult to collect and curate personal data collected by activity-tracking apps.
The authors acknowledge that it is difficult to collect accurate information on exposure retrospectively, that the study is small, and that it is limited to occupational exposure.
However, in practical terms, it is difficult to collect fresh urine from individual animals in the field, especially from small animals.
Large-scale sensor networks impose energy and communication constraints, thus it is difficult to collect data from each individual sensor node and process it at the sink.
Mechanical energy appears everywhere in the nature, road vibration energy created by the vehicle will not only do damage to pavement structure, but also is difficult to collect.
The advantage of this system is that it can make use of the low grade solar heat, which is difficult to collect or use in normal solar water heating system.
In vivo digestibility determination in shrimp is a challenge because these animals are coprophagous, benthic and slow feeders and the small amount of feces that they produce is difficult to collect.
But it's important to specify that information about local signs of infection is difficult to collect in a retrospective study.
The most essential data is difficult to collect reliably and persistently, and nearly impossible to parse, scale and package without diluting or reducing its accuracy.
However, it is difficult to collect sensitive electrochemical signals for the DNA electrochemical sensor-based electrochemical oxidation of nucleobases (primarily purine) [56].
