The phrase "are more difficult to perform" is correct and usable in written English.
It can be used when discussing tasks, actions, or activities that require a higher level of skill or effort to execute.
Example: "The advanced techniques in this project are more difficult to perform than the basic ones."
Alternatives: "are harder to execute" or "are more challenging to carry out."
The dynamic tensile tests of concrete are more difficult to perform and there are limited results available.
However, analytical investigations and computations for diffusions with so-called heavy-tailed ergodic distributions are more difficult to perform.
Another limitation is that measurements of arterial CO2 tension (PaCO2) are more difficult to perform under such experimental conditions in rats compared to humans [92].
Analysis of the effects of the mass loading, and the resulting electromagnetic asymmetries, at Mars and Venus are more difficult to perform than the test seen in Fig. 3 because the planets form impenetrable surfaces and have conducting ionospheres.
Literature reports that the pointing tasks from an imagined perspective misaligned with the body are more difficult to perform because the body SFR conflicts with that of the environment (Presson and Montello 1994).
Third, and most importantly, therapeutic trials are more difficult to perform.
The SDS-PAGE analysis confirmed the MALDI-TOF data but was more difficult to perform.
Our results endorse the fact that US is more difficult to perform in overweight patients, but that it is still of great value.
Mathematically rigorous analysis of the regularization method for the PMEs, which would be more difficult to perform, is beyond the scope of this paper and will be addressed elsewhere.
In a collaborative environment, it is easier to share findings and communicate ideas than on a single-user desktop computer, but it may be more difficult to perform individual analyses due to interference [26].
Abnormal elongation of the canal is defined as greater than 12 mm in length (Fig. 6) [9], however this measure is more difficult to perform and for this reason is considered a less reliable criterion [10].
