'effectiveness varies' is correct and usable in written English.
It can be used to indicate that different levels of effectiveness exist, depending on the situation or context. For example, "The effectiveness of different learning methods varies depending on the individual student."
Although MSCs have been proven to be very effective in inducing immunosuppression in both animal models and human trials, their reported effectiveness varies significantly.
The vaccine's effectiveness varies by population, too.
Washington finances 70 nonprofit groups, and their effectiveness varies greatly.
Evidence on the effectiveness varies by the condition being treated, the drug used and the injection technique.
Mr. Bennett rightly points out that there are different kinds of psychotherapy and that the evidence base for their effectiveness varies.
The new report, appearing in The Journal of the American Medical Association, reviews data from previous trials on two types of drugs and finds that their effectiveness varies according to the severity of the depression being treated.
Small-molecular-weight MMP inhibitors were indeed effective for inhibiting proteolytic activity in SF, but their effectiveness varied greatly among patients.
Third, small molecular weight inhibitors were very effective in inhibiting MMP activity in RA SF, but their effectiveness varied significantly among patients.
Building separation and building setbacks were effective in reducing personal air exposures in canyons with perpendicular wind, although their effectiveness varied with different configurations.
Opinions on the trip's effectiveness vary among participants, but in the six months since, not a single foreign company has been employed in Japan's cleanup, according to the trip's participants and Japan's Environment Ministry.
The heat exchanger effectiveness varied between 0.620 and 0.901.
