The phrase 'decisively affect' is correct and usable in written English.
It can be used to describe a strong or significant influence on something. For example: The new tax incentive had a decisively positive effect on small business owners.
That is that changes in the dynamic system behavior can decisively affect the KLT results.
The outcome, needless to say, could decisively affect the presidential election.
But it isn't only Argentina whose players decisively affect Champions League games.
Furthermore, the slenderness and the flexural crack pattern have been found to decisively affect the critical shear crack formation, hence the shear resistance of continuous beams.
This suggests that dramatic examples of government dysfunction should perhaps be added to the short list of events that decisively affect presidential approval.
The U.S. is in the midst of the most important presidential election campaign since World War II, and if there has ever been a time when women voters had the opportunity to decisively affect the nation's future, it's now.
In addition, the heat treatment atmosphere decisively affected the κ-α-transformation.
He was decisively affected, in 1957, by an encounter with the art of the French avant-garde paladin Yves Klein.
Furthermore, Italian architectural style was decisively affected by the fact that brick not stone was the most common building material and marble the most common decorative material.
The expansion of Rome into the Balkan Peninsula in the 3rd and 2nd centuries bce decisively affected the evolution of the Geto-Dacians.
Krieger Vasena was removed, but the Onganía administration was unable to agree on an alternative economic policy, and the Cordobazo decisively affected the political climate.
