Studies on planets without parent star I am interested in studies and/or publications about planets that wander in space and are not orbiting around a star or its remnants. These dark and frozen worlds should've got ripped away from their respective system by a large passing body (another star? passing black hole) or any other astronomical event (which one?).

I am curious, whether there exist statistical estimates on the number of such planets and the rate at which they are produced. I am also interested in reading about the statistical models that can produce those estimates and encode researchers' beliefs. If there are no such models, why?

A provoking thought: According to Greek, such planets are really fitting their name πλάνητες ἀστέρες (planētes asteres, "wandering stars") or simply πλανῆται (planētai, "wanderers"). It is frightening to imagine such world that drifts deeper and deeper into empty space. Forever.
It would be more accurate to call them surveys than studies.   A study looks at an object in detail.  A survey counts and categorizes objects.

Rogue planets are enormously difficult to see, and only a few very large ones have actually been observed directly.  For the largest rogue planets, it's unclear if they actually are ejected planets as opposed to failed stars. 

This article discusses rogue planets in pretty good detail.  

In 2011 the MOA survey was published, estimating a high number of Jupiter sized Rogue planets, perhaps outnumbering stars. 

More recently the OGLE Project estimated about 1/10th that many rogue Jupiters, which is more in line with statistical predictions.   Smaller planets are more likely to get ejected from their solar-systems than larger ones.    Rogue Juiters outnumbering stars seemed unlikely and my understanding is that most scientists are more in line with the OGLE estimate than MOA.

Both estimates rely on microlensing which amounts to very small and temporary changes when rogue planets drift between the Earth and a star.  There's a fairly large margin of error for both surveys.

NASA's WFIRST telescope should answer a lot of questions on rogue planets.   It's not expected to be launched till the mid 2020s though.