OK. There are a few questions here!

Reliability of Ancestry.com

Is it reliable? Their documents certainly are. Those documents are census, marriage, birth, and death records. Land deeds and immigration records. Military records, obituaries, city directories... I could go on, but I think you get the point. These are all primary source documents which they have digitized and made available, and it is probably the single most comprehensive source out there for this kind of stuff. Certainly, you can go about it on your own, but that requires a lot more work! Looking through tons of microfilm, or browsing through old records in a church basement. Ancestry.com puts it all in one place, and I don't know of another group which has more information collected (Despite all this praise, I am not affiliated in any way with the site! In fact I'm not even a paying member! I just use the small amount of free services they make available, and occasionally use a throwaway email to get a free trial when needed). Anyways, point is, that information is reliable. They aren't making up the records. But those records are only as good as the person using them, which brings us to the part where Ancestry.com ISN'T reliable.

People want to believe they have all kinds of cool ancestors, so are inclined to go with questionable information, if not make things up out of whole cloth. That feature in the commercials where you have your family tree, and there is the little green leaf, which shows you other people's trees? That feature is NOT TO BE TRUSTED! Don't trust any of the other trees you come across that your own overlaps with, unless you confirm it through your own, independent research. And even then, the further back you go, the more cautious you should always be.

Are You In Fact Descended From the King of Norway

Yes. That is a fact (Well, probably not from the current one unless your parents are lying to you, but from a King of Norway).

"Wait, How The Hell Do You Know That!?"

Science, History, and Math of course!

First, the science.

There have been a number of studies in this regards, and I'll be citing this recent one from 2013. To quote from the abstract:

We find that a pair of modern Europeans living in neighboring populations share around 2–12 genetic common ancestors from the last 1,500 years, and upwards of 100 genetic ancestors from the previous 1,000 years. These numbers drop off exponentially with geographic distance, but since these genetic ancestors are a tiny fraction of common genealogical ancestors, individuals from opposite ends of Europe are still expected to share millions of common genealogical ancestors over the last 1,000 years.

Basically, everyone with recent European ancestry is almost certainly related within the past 1000 years. There are some regions where the relation is small - Spainsh populations being probably the most notable example of a very small connection to the rest of Europe - but is pretty much proven by studies of human genetics. I've read some studies that postulate the most recent common ancestor of every living European right now was alive only 600 years ago! That is a little to recent to be trusted probably, and I've never seen anything to really back it up, but it is pretty much agreed that, at the very least, if you have European ancestry, you can count Charlemagne as your great-x-X grandfather. Or in your case, Olaf II Haraldsson. But I'll stick with Charlemagne - or Ol'Grandpa Charlie as I call him - because he is better known.

"The Science is There, But I'm Having a Hard Time Believing You!"

First, we can look at how quickly his children/grandchildren/etc. spread across Europe. By early in the second millennium, they had found their way through marriages and inheritance into the thrones all over Europe, not to mention countless lessor titled positions who were related to him. Those whose descendants remained notable, we can continue to track through very good quality records, which is why it is very easy to show exactly how, say, Queen Elizabeth II descended from Charlemagne. But the only difference between her and the average Joe is that there is that chain of notable people we can track, while for common people, records of their existence are mostly just a thing for the past few hundred years - or at least I should say, the likelihood of those records surviving and making enough sense to trace heritage back all that way. Seventh sons of seventh sons, so to speak, begin to lose social status over a number of generations, and that's where the records get fuzzy, and generally don't exist any more. To use as an example, Karin Vogel is (or rather, was in 2011) 4,972 in line to the British throne, being the very bottom of the pecking order for the line of succession figured out by researchers who tried to figure out every qualifying descendant from Sophia of Hanover (George I's mother) on wards, Sophia being her great-x-8-grandmother. She lives in Germany and works as a therapist for elderly people with chronic pain. Because this all was kind of recent, we can at least track back, but this kind of generational decline would simply fall out of the (surviving) written record had it been 1000 years back. Those now who can figure out their royal roots can do so because of quality record keeping in recent times, which trace back to someone notable enough to have a record that goes back further, but simply put, we probably aren't going to be tracing a genealogical branch back through one thousand years of peasants. John the dirt-farmer in 11th century England just doesn't have his name in many books.

Now if we go before Charlie, stuff gets fuzzier. For even the most notable of notables, records get fuzzy. If you want proof of this, go back far enough in QEII's tree, and it jumps to some semi-mythological Norse heroes (Ragnar Lothbrook!) and eventually some Norse deities like Thor. Anyways though, we can kind of piece together some evidence, which some researchers try to do. The best documented would possibly be this guy, Flavius Afranius Syagrius, who was consul in the late 300s CE. When I say best documented, understand that I mean our best bet, as needless to say, establishing records for that is REALLY REALLY REALLY hard. I digress though, Flavius is considered a very likely ancestor of Charlemagne for a traceable line of descent, which is part of a larger goal, known as "Descent from Antiquity", kind of the holy grail of genealogy, where researchers want to document a reputable, traceable line from someone living today all the way back to ancient times. The other candidate is this guy, Anastasius, who was also a consul in the 500s, and himself descended from Valentinian, although I guess no one is clear how. I won't pretend to be super well versed in the whole thing, so you can read up more on DFA here. (Also note this is just Europe. In China, there is a purported line from Confucious, but I know even less about that).

Cool, I Think I Get It... But You Have So Much Free Time, Do You Mind Maybe Giving Me One More Way to Understand This?

OK. Like I said, this also makes sense mathematically. Going back to good old Charley, he died in 814, or 1200 years ago. A generation is roughly 25 years, so if we divide, we get 48 generations between now and his death. Now, if we assume 2 children per each person in the tree (Charlemagne has 20, at least, for the record), we can express this as 2^48, which equals 2.8147498e+14, or 281,474,980,000,000. That is 281 Trillion. That number is the theoretical number of living descendants he would have, if every kid went on to have two kids themselves. Well, last I looked, living space is tight, but not that bad. That comes out to roughly 40,211 theoretical people for every actual person on this planet right now. If we exclude Asia, and just talk about the European descended population (lets say... 1.5 billion), that number goes up quite a bit to 187,650!!! So where are all these extra people? Well, if you go back far enough in your family tree, you start getting what I call "closed loops", although the proper name is pedigree collapse. Basically, it is people showing up multiple times in your family tree. So practically speaking, instead of nearly three hundred trillion descendants from Charlemagne walking the Earth, there are a lot fewer, who we just count a few thousand times each. As I understand it (and assuming my ballpark numbers are actually correct), on average every person of European decent can trace their heritage back to Charlemagne through 187,650 different paths in their family tree, because depending how you look at it, they count as that many different people.

And of course the math works in reverse. We assumed two kids per person, but assuming two parents per person (reasonable assumption, no?) that also means 2.8147498e+14 ancestors at the 48th generation, at a point where the world population was comparatively tiny to what it is now.

Now of course, a word of caution, just because it is certainly true that you can claim almost any given person alive in Europe 1000 years go to be in your family tree, that doesn't mean you should accept some family tree you see on Ancestry.com as fact. Like I said at the beginning, their documents are good, but it really depends on how the researcher used them.

I Skipped to the End. Whats the TL;DR?

Due to genetics and the power of exponential growth, no one is special. We are all royals if you go back far enough. The only difference between you and the Queen is the paper trail.

One of the main sources on there that you can get information from is other people's family trees, so if you use those then you're trusting some stranger who is distantly related to you to not claim you are both descended from Julius Caesar. You don't have to use public trees, but many people do out of a desire to get back further on their trees. If you think about it, the further back you go, the more people you are related to, so there is a better chance that you might find a public tree with helpful information. But the further back you go you also lose more reliable sources like birth records. I did my tree and found it increasingly exciting as I went further and further back. 1800! 1500! 1100! Soon it started to claim I was descended from Marcus Aurelius and later Joseph of Arimethea. But realistically, if I hadn't used questionable public trees as a source, I probably would have made it ten or so generations back at most, which is actually still a lot. Basically you would just have to wisely judge your sources, since Ancestry.com offers plenty of good and bad ones.

Edited:grammar.

The documents themselves are primary sources, but the family trees constructed by others are not, and sometimes make logical leaps without the necessary supporting documents, based merely on similarity of names or dates.

Thanks for the feedback. As an added question, are there any quality free genealogy databases available online?

I started my family tree without using a paid site like Ancestry. I got a couple of generations back before I subscribed to it.

It does have copies of a lot of primary sources like census data, school yearbooks, city phone directories, SSDI, state birth and death records, old newspaper articles, etc.

Some states don't allow their records online and only provide indexes which often don't contain enough data for you to know if this is your ancestor or not.

The one thing I hate about ancestry is that they have a inefficient search model. For example, if I put in my ancestor's first and last name, place of birth and date of birth, Ancestry will return thousands of records that have nothing to do with my ancestor. They will list people from other states and with other first names before people from the state I indicated and with the name I indicated. I believe they do this so it takes longer to find all the data they have access to and you have to keep your subscription longer. The other reason may be that a lot of records had to be transcribed in order to be searched resulting in a lot of transcription errors. Census takers also spelled names wrong or got the name completely wrong, so Ancestry gives you results that match each of your search terms individually as well as all the terms together.

The other thing I don't like about Ancestry is that they automatically renew your subscription so if you do subscribe for a year, go into your account and immediately cancel your subscription. You will still have access for a year, but it won't auto-renew.

There's nothing at all wrong with Ancestry as a repository for primary sources. The problem in reliability comes from user error. In my experience, one of two things commonly happens: either 1) a person just copies the family tree of someone else who has been researching that family, and inaccuracies get perpetuated, and/or 2) they work forward from a family legend, e.g. "we're descended from the king of Norway in the 1600s", rather than taking the accurate path of starting with a verifiable recent relative and working backwards from there.

For people who are acquainted with how to do good research, though, it's an invaluable resource. My personal policy is just to never look at anyone else's family tree or user-submitted materials, with the sole exception of photographs. Even then, I won't include a photograph in my records for a given person unless the photo can be verified to really be that person. For example, if I were to find a photo uploaded by a random person claiming it was my great-great aunt, but I had no other photos of her and nobody alive knew what she looked like, I would probably not accept that photo. Conversely, I found some photos that were uploaded by a user who had identified my great-grandma and other relatives of my still-living 97 year old grandpa. Since he looked at the photo and was able to verify that they were in fact his family members, I'll include those.

The UK site has full access to census data which goes back hundreds of years, and then court and church records extending further back.

If you painstakingly construct your family tree you can use solely primary evidence. The problem comes with people using multiple names, changing their birthdates and places etc. It is very easy to make mistakes.

For example, my family name is Morgan and my family comes from Glamorgan. There are literally hundreds of David/John/Trevor Morgans born in the same town every year. It is hard to track the correct one! Church records are even less accurate. I have one ancestor in the seventeenth century who is recorded as living past 100 years, according to parish records. Maybe he really did, but I'm inclined to believe it is a clerical error.

Once you start importing chunks of family trees built by other people you risk importing their mistakes or flights of fancy.

Your friend could well have accurately traced their family back to Scotland and Jamaica. The Norway bit sounds like they have imported other people's tenuous trees.

Just a quick tip I found using the site. Although there are primary sources, they have been digitized to make it easier to search. There can be errors when going from hard-to-read, quickly written cursive to digital. For example, there is a lower case g in my mother's maiden name, but I found some ancestors where this g had been changed to an s. If you didn't look carefully below the line, it does look like an s. For the most part ancestry has been pretty cool, and they offer free looks into certain documents throughout the year. This is just something for the hardcore family researchers to take note of.

Follow up question. I've always imagined Ancestry.com to be more of a tool for the western world to trace their geneology. Is there any information on how accurate their data is for someone of Asian decent, if there is any information at all?

I've used Ancestry quite a lot for my own family research and doing research for others.

While the site gives you access to a wealth of information, it's how you use it that determines the reliability of the tree you're making. It is very possible, and fairly common, to simply incorporate everyone who shares your surname into a tree and create something that bears no resemblance to fact. This sounds like what may have happened with your friend.

The key is to cross-reference everything and critically examine any suggested "leaf" connections to other people's trees.

Does anyone know of any similar sites that work for non-Euro-Americans? I'm skeptical that there's any Soviet Union paperwork to be found online...

How is their coverage for non-english-language documents? I have some family who lived in Brazil and in Spanish North America that I'd like to try to trace further back if possible.

But can it trace back Asian lineage, or just European ones?

I do want to point out to consider what the source documents actually are. Having been a census-taker myself, I've seen how rushed things can be and how problematic people can be to get it done. You can get dates, names, ethnicity completely wrong by simple error. I'm not saying anything is by default wrong, but you've got to grind through a lot of mistakes. And, this doesn't stop with censuses, as there were sloppy priests in the 1600s, just like today.

i suppose it goes to show that you really have to engage brain before speaking

Is it true that in maternity wards of the past, it was known that occasionally some parents would switch babies with someone elses if theirs was better looking? I'm sure I read that somewhere long ago, and its always made me wonder how accurate these ancestry databases are if baby switching was a thing. Someone please tell me that never really happened.