The phrase "favorable factor" is correct and usable in written English.
You can use it to refer to anything that could have a positive impact on a process or an outcome. For example, "Having a flat tax rate was a favorable factor in the country's economic growth."
Isn't that a favorable factor for Mr. Romney?
But Houston's problems can be traced to one surprising — and usually favorable — factor: the analytically advantageous three-point jump shot.
But recent sharp declines in crude oil prices are regarded as one favorable factor for the Japanese economy, which is highly dependent upon imported oil.
Another favorable factor is the excellent rice harvest this year, with the expectation that South Vietnam will be self‐sufficient in rice by 1971.
The high surface-to-volume ratio is a favorable factor to obtain high specific capacitance and excellent rate performance.
The favorable factor is that population growth is slower than at most points in the recent past, which puts somewhat less pressure on the job market.
Mr. Gordon of Northwestern asserts that the Clinton administration benefited from an "extraordinary coincidence" of favorable factors that kept the dollar strong even as investment flowed in.
Overall, the Nikkei index rose 17.5percentt in the last three months as a number of favorable factors converged to attract domestic and foreign investors.
Constructing something as complex and sensitive as a Middle East peace agreement requires a confluence of favorable factors, including propitious timing, compatible negotiators and a high degree of mutual trust.
"At that time, President Obama directed the Department of Homeland Security to exercise prosecutorial discretion on certain people who could legally be deported. . . and refrain from deporting them if they have more favorable factors than negative factors in their life," Bennett said.
The favorable factors that facilitated the whistlers prior to the sunset are investigated in terms of source lightning characteristics, geomagnetic and background ionospheric medium conditions.
