Intro to Search Engine Optimization
Crawler-Based Search Engines Crawler-Based search engines, such as Google, create their listings automatically. They “crawl” or “spider” the web, then people search through what they have found. If you change your web pages, crawler-based search engines eventually find these changes, and that can affect how you are listed. Page titles, body copy and other elements all play a role. Human-Powered Directories A human-powered directory, such as the Open Directory, depends on humans for its listings. You submit a short description to the directory for your entire site, or editors write one for sites they review.
A search looks for matches only in the description submitted. Changing your web pages has no effect on your listing. Things that are useful for improving a listing with a search engine have nothing to do with improving a listing in a directory. The only exception is that a good site, with good content, might be more likely to get reviewed for free than a poor site. The Parts of a Crawler-Based Search Engine Crawler-based search engines have three major elements.
First is the spider, also call the crawler. The spider visits a web page, reads it, and then follows links to other pages within the site. This is what it means when someone refers to a site being “spidered” or “crawled”. The spider returns to the site on a regular basis, such as every month or two, to look for changes. Everything the spider finds goes into the second part of the search engine, the index. The index, sometimes called the catalog, is like a giant book containing a copy of every web page that the spider finds. If a web page changes, then this book is updated with new information. Sometimes it can take a while for new pages or changes that the spider finds to be added to the index. Thus a web page may have been “spidered” but not yet “indexed”. Until it is indexed – added to the index – it is not available to those searching with the search engine.
Search engine software is the third part of a search engine. This is the program that sifts through the millions of pages recorded in the index to find matches to a search and rank them in order of what it believes is most relevant. Major Search Engines: The same, but different All crawler-based search engines have the basic parts described above, but there are differences in how these parts are tuned. That is why the same search on different search engines often produces different results. Now lets look more about how crawler-based search engine rank the listings that they gather. How Search Engines Rank Web Pages Search for anything using your favorite crawler-based search engine. Nearly instantly, the search engine will sort through the millions of pages it knows about and present you with ones that much your topic. The matches will even be ranked, so that the most relevant ones come first. Of course, the search engines don’t always get it right. Non-relevant pages make it through, and sometimes it may take a little more digging to find what you are looking for.
But, by and large, search engines do an amazing job. As WebCrawler founder Brian Pinkerton puts it, “Imagine walking up to a librarian and saying” ‘travel’. They are going to look at you with a blank face”. Ok- a librarian’s not really going to stare at you with a vacant expression. Instead, they are going to ask you question to better understand what you are looking for. Unfortunately, search engines don’t have the ability to ask a few questions to focus search, as librarians can. They also can’t rely on judgment and past experience to rank web pages, in the way humans can. So, how do crawler-based search engines go about determining relevancy, when confronted with hundreds of millions of web pages to sort through? They follow a set of rules, known as an algorithm. Exactly how a particular search engine’s algorithm works is a closely kept trade secret. However, all major search engines follow the general rules below.
Location, Location, Location… and Frequency One of the main rules in a ranking algorithm involves the location and frequency of keywords on a web page. Call it the location/frequency method, for short. Remember the librarian mentioned above? They need to find books to match your request of “travel”, so it makes sense that they first look at books with travel in the title. Search engines operate the same way. Pages with the search terms appearing in the HTML title tag are often assumed to be more relevant than others to the topic. Search engines will also check to see if the search keywords appear near the top of a web page, such as in the headline or in the first few paragraphs of text. They assume that any page \relevant tot the topic will mention those words right from the beginning. Frequency is the other major factor in how search engines determine relevancy. A search engine will analyze how often keywords appear in relation other words in a web page.
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