I've never seen a ghost, but my sister claims she did when she was little. This is one of those "I have to see it myself" type of situations. I'm leaning towards them being real because hundreds of millions of people have seen ghosts according to various surveys and I do place some value on anecdotal evidence.
First, I'll start by saying I do not believe in "ghosts" or anything that could be considered one. We would have to do lots of defining on what a ghost is to really sort out what people mean, but that's another discussion.
I find this interesting for you to say for two reasons. The first is that your taking into account a large group opinion (which I have seen you very often disregard), and it is also anecdotal. The latter part is a problem. As I have told you before I actually put a heavy weight on group opinions. This is one case though where group opinion shouldn't be considered. In particular because it's asking a general population, not a specific group.
Anecdotal evidence is widely regarded and known to be unreliable. While it can be used to infer things and lead us in a direction for further investigation, it's rare for it to be used for something iron clad. Especially for something as solid as proof of existence.
I actually believe the concepts of ghosts to be very similar to what microbial diseases used to be considered many centuries ago. In many communities it was thought to be due to possession, spells, etc. or some sort of magic. Part of this is because for the time the explaination was easy and made sense. Further, it could be expanded upon enough to create tools, rituals and whatnot to make it seem like something was being done to affect it. The affects were often minimal, but it was enough to offer some sort of apparent support of it. There was never anything solid about it. We now know definitively what was behind this: microbial diseases.
With ghosts and anything that can fall under that catagory, we really don't know what they are. That being said, it's extremely unlikely that they are as people assume them to be, even similarly to that. Why? Because there remains unexplored rational and solid causes behind many of these. The biggest likely culprit behind ghosts is the human brain. Hopefully this will be sorted out in the next several decades due to the
BRAIN initiative. We know that psychology is incredibly complex and poorly understood because of how poorly understood the brain is. However, we do know that the brain can and does cause delusions, hallucinations, and other anomolies in people. Sometime rather healthy people as well (meaning they don't have to have some sort of ailment or psychological disease). Based on this, it stands to reason that ghosts could in reality be a complex interplay between the brain, how it processes information from the environment, and the circumstances around the person at the moment. There could be external environmental factors as well. Also, ghosts often impart emotion, and very strong emotions at that in individuals experiencing them. They are very often negative emotions, but can be positive. Either way, emotions are
powerful and illogical influencers.
The fact of the matter is that ghosts, spirits, etc. don't have any solid evidence to support their existence as they are commonly defined by people. The major lynchpin being that they are assumed sentient in some manner. It's rather illogical to take this poor assumption as opposed to investigating or regarding possibilites that have solid backing in reality. This is quite similar to
occam's razor which state that, in the absence of a proveable answer, the correct one is often the simplest based upon what we know. This is how science often begins and works through solving problems.