Possible explanations for the meaning and the ending Haunting of Hill House

What is the haunting in The Haunting of Hill House (Shirley Jackson, 1959)? Some possibilities: The House is actually haunted by the spirits of those who died there or those whose led miserable lives there; the "haunting" is actually the product of the rich imagination of the quartet  of observers who agree to spent a week or so in the House; the "haunting" is a trick that some of the visitors have concocted to torment the others (or perhaps only one other, the central character, Eleanor) and observe how they react to the door knocking etc.; the haunting is all in the head of the main character, Eleanor. Perhaps there are other possibilities, too. Jackson never definitively states what the haunting represents, which in a way is the strength of this novel. She turns the ghost story on its head; most if not all readers are skeptics, and we watch w/ benign amusement at Dr. Montague's scientific approach to the haunting (measuring the area covered by the cold patch of air, for ex.) and in particular by Dr. M's wife who believes inmessages received from the "planchette" - but if we don't believe in ghosts, then what sense does the novel make? And why are we scared when reading it (I wasn't truly scared on re-reading, but remember being frightened reading it in my youth, particularly reading it late at night in a run-down Baltimore rowhouse)? In the end - obvious spoilers here - the group asks the obviously disturbed and troubled Eleanor to leave, and as she does so she plows her car into the basse of the turret, killing herself; in a sense, then, the house is truly haunted - the very idea of specters and spirits is enough to attract mentally unstable visitors and to hasten them toward death. The ending does seem to tip the novel one of the possibilities outlined above: It's all in her (Eleanor's) head, perhaps. And of course E's plight is the most moving and troubling aspect of this novel - her sense of having no friends, no home, no life, of being the one observed, the object of the snide and condescending comments of others. She is the true outsider - much like Jackson herself, as we now know - and in that sense Hill House isn't a ghost story at all but a story of loneliness. As the first and last pages note: whatever haunts Hill House "walks alone."