decisive factor in...

[ragan]

Can I say these in English?
For me, long-term memory is the decisive factor in this whole thing.
For me, long-term memory is the decisive factor in the whole study.

 

[ragan]

What about vice-versa?
In this whole thing, long-term memory is the decisive factor for me.
And what if I use "at" instead "in"?
At this whole thing, long-term memory is the decisive factor for me.