Formulas in Sandy Petersen's Cthulhu Mythos are similar to Rituals with a few key differences. Like Rituals they can be cast without a spell slot, and you can also use spells slots to cast them (generally more quickly, or potentially at a higher level). Unlike Rituals Formulas require Skill checks to cast, carry penalties for failure (and sometimes success), and can be cast by anyone, even non-casters.
The fluff around formulas contains the following:
Such a spell draws upon universal principles that allow them to function even for non-spellcasters. Formula magic uses science and mathematics beyond most mortal understanding to power the casting of highly specialized spells. Most formulas are held only in shunned books and in the minds of dangerous eccentrics who know more than is safe about the secrets of reality. Generally, these spells depend upon knowledge of the alien sciences, extradimensional mathematics, and incomprehensible iconography of the cosmic forces underlying the Mythos.
The consensus seems to be that Rituals are able to be counterspelled at any time during their casting; I don't see a reason why Formulas would necessarily be different, but there are enough differences that I'm left unsure.