The phrase "is not frequent" is correct and usable in written English.
You can use it to describe something that does not occur often or is rare.
Example: "The occurrence of such events is not frequent, making them noteworthy when they do happen."
Alternatives: "is rare" or "does not happen often."
While violence is not frequent, conflict is "fairly commonplace," according to a 2005 MSPB survey.
Today the Metro is very crowded, and bus service is not frequent enough.
Goal oriented sedation is standard in the management of critically ill patients, but its systematic evaluation is not frequent.
The reason for this unease, even after the terrorist attacks of September 11 , 2001 is not frequent threats to American security.
As in any penal system in the world, there are reports of mistreatment, but it is not frequent and has affected only a small percentage of the population.
Selecting water saturation in the mixed form as a primary variable, which is not frequent in the geotechnical engineering, could produce convergence problems in transition from saturated to unsaturated regimes, but in other conditions show good convergence and also mass balance properties.
It is not frequent, but they can also be multiple.
This analysis is performed periodically and therefore the reorganization of application data is not frequent.
This finding is a highlight and is not frequent for other materials subjected to HTC under similar conditions.
This suggests that whilst lesional calcification is a not uncommon finding in DSRCT, it is not frequent enough a finding to be characteristic.
Scleritis is associated with systemic disease in approximately 50%% of cases [2]; however, it is not frequent in IgA nephropathy (IgAN).
