In the last lesson, I started to give you some visibility into how links play a role in search engine rankings. In this lesson, I plan to show you more about how links are valued by search engines, and start to give you some idea on how you can get measurements of link value for yourself. To start, I want to talk about basics of PageRank. Now, you saw some of this in one of the earlier courses in this specialization. But the way it works is that each page upon creation is given a certain tiny amount of PageRank. Then those pages are able to accrue additional PageRank based on every link that points to them. To get an idea of how this works, say your page has PageRank value, call it X. Certain percentage of that can be voted by that page in links to other pages, we'll call that quantity f of x. The way that works is the PageRank is split among the outgoing links. In this case I'm showing a page with f of x being the PageRank that it's voting out and it gets split between these two pages. Now, to be fair, in practice, it may not be exactly 50 percent to each of the two links. It might be slightly different based on where a link is placed on a page, but for purposes of this discussion, you can just imagine it's exactly equal. So far, the picture looks pretty simple. However, here's where it starts to get a little bit more complicated. Imagine you have page A over here on the left that links to pages B and C shown on the top right and bottom right. So far it's straightforward. But now page B links back to page A. Now the math starts to get very complicated because page A has voted some PageRank to page Y, which votes some of its PageRank back to page A, which makes page A's PageRank go up. But it's voting some of that to page B and it gets very iterative and becomes a very complex calculation to figure out what the final page rank values are. This is a problem that search engines solve all the time. In fact, it gets a lot more complex because the real web looks something more like this. In fact, even this diagram that I'm showing you now is very simplistic. The real web consists of hundreds of trillions of web pages all linking to each other, this massive array of links going back and forth, Google and other search engines are able to resolve those calculations and figure out the PageRank value for each of the pages on the web. One more thing that I want to talk about, which is this notion of the Google toolbar, which historically has had this PageRank meter on it, which supposedly shows you the PageRank for a web page. Couple of important points you should know about that. First of all, it's not the same as real PageRank what has been shown there, it's actually a logarithmic representation of the real number PageRank. What that means is that if you have something which Google calls PageRank six, it's, in theory, something like ten times more important than a PageRank five-page. This is going to be important point that I'm going to emphasize more as we go through this course. It's assigned to web pages, not sites. There's no such thing as a PageRank for a domain or a site. That's an important thing for you to remember. However, this PageRank that we see in the Google toolbar is no longer updated. The metric you see there is obsolete. The only reason that I brought you through all this is that once you understand that you can't rely on the Google toolbar as a way to get this measurement. Now I want to give you an example though, now that we've gotten this far. Ask yourself a question, if you have 10,000 PageRank, two links and one PageRank seven link, which of those seems more valuable? Ten thousand links is an awful lot of links or second challenge question for you, if you've got 300 PageRank, five links, are they more important or more valuable than five PageRank seven links? Well, here's the true story; it turns out that one PageRank seven link is actually worth quite a bit more than 10,000 PageRank two links. You can see the math here, since one PageRank seven is worth 10 PageRank 6, and that's worth a 100 PageRank 5 and so forth. One PageRank seven link is actually, in theory, worth a 100,000 PageRank two links. Remember, I said earlier on, one link can be worth a million times more than another one. This is just really an illustration of that. You can see the same thing with my example over here on the right, where five PageRank seven links is worth more than 300 PageRank five links. Now keep in mind, I keep emphasizing that one PageRank 7 or one PageRank 6 is 10 times more valuable than one PageRank step down. That's just an approximation. But you should keep it in mind and get across this notion that one link can be worth a heck of a lot more than another one. As I mentioned, Google no longer updates PageRank that you can see in the toolbar, but they do still update the real PageRank and links remain a major part of their algorithms. The three main tools I like to use to get information on links are Open Site Explorer, Majestic SEO, and Ahrefs. Each of these are tools that you can use to get information about who's linking to your site or the sites of your competitors. With them, they offer metrics similar to Google's PageRank, which you can use to get some idea of how to value links. For example, MOZ's Open Site Explorer has a metric called Page Authority, which is their approximation of Google's PageRank. They calculate it in a method very similar to what Google is believed to be using to calculate PageRank today. MOZ also has a metric called Domain Authority, which is an estimate of a site's overall authority. Or you consider it as the authority of the entire domain. Remember, I said earlier, Google does not have a such a thing as domain PageRank or PageRank for an entire site. But MOZ does have their own calculation along those lines. For Majestic SEO, they have their own calculations called Trust Flow and Citation Flow. These are also meant to be calculations of the importance of a link. They're not calculated by the same type of algorithm as Google uses, but they still can be very valuable in helping you understand when one link is more important than another. Then Ahrefs also has their own measurements, one is called URL Rank, and then they also have a domain rank, as you can see here. Very similarly, these are used to help you understand which links are most important. One more thing to cover about links, which you should really have in mind when you're looking at the value of the link, and that's how relevant it is. If you have a page about used Ford Mustangs and you'd get a link from a casino gambling site, well, that's not very relevant, is it? That's pretty low value. Contrast that with a link from Car and Driver Magazine or Ford vehicles.com. Well, that last one is a made-up domain. But anyway, those links clearly seem a lot more relevant. You're getting a vote here in sense from an authority on a topic, Car and Driver Magazine in this case to your page on the same topic. Clearly, that should carry more weight, and the search engines understand that. Another thing to think about is what are the authoritative sites in a given market space? Here I'm talking about topical authority. Imagine you have this picture which is all websites about used cars. Some of them, the three that I've highlighted in the blue circles, they seem to be getting links from a large number of sites in the space. Those are the ones that you might think of as being the authorities on that topic. Links from those sites probably carry more value as well. If you back up and try to have a way to think about how you would assess a link's value, here's some things to think about. First of all, I do like to use Domain Authority and Page Authority from MOZ as one basic measure. But you can certainly use the Majestic and Ahrefs measurements as well. As I've said several times, keep in mind, one link can be worth a million times another. That's going to be really important as we talk through content marketing. Topical relevance is also a very large factor. The more relevant to link is the better. Then just for me, I'd like to place the most weight on Domain Authority as a metric. To me, that says the most about the value of a link. So if a page on a website links to me and that website's Domain Authority is higher, I like that. Then my second theory metric is Page Authority. That's the one I like to look at next. In this lesson, we talked about what makes one link more valuable than another. We also talked a bit about how you can get a determination of a link's value for yourself. In the next lesson, we'll start to talk about link building, which is what SEO professionals use to focus a great deal of energy doing to obtain links to their sites. I'll also provide you some guidelines that will help keep you from going about building links to your site in a manner that search engines don't like.