Hello
I am a student carrying out a study to enhance a search engine's existing algorithm.
I want to know how I can evaluate the search engine - which I have improved - to quantify how much the algorithm was improved.
How should I go about comparing the old and new algorithm?
Thanks
In order to evaluate something, you have to define what you expect from it. This will help to define how to measure it.
Then, you'll be able to measure the improvement.
Concerning a search engine, I guess that you might be able to measure itsability to find things, its accuracy in returning what is relevant.
It's an interesting challenge.
You have to clearly identify positive and negative qualities such as how fast one gets the answer they are seeking or how many "wrong" answers they get on the way there. Is it an improvement if the right answer is #5 but the results are returned 20 times faster? Things like that will be different for each application. The correct answer may be more important in a corporate knowledge base search but a fast answer may be needed for a phone support application.
Without parameters no test can be claimed to be a victory.
This is normally done by creating a test suite of questions and then evaluating how well the search response answers those questions. In some cases the responses should be unambiguous (if you type slashdot into a search engine you expect to get slashdot.org as your top hit), so you can think of these as a class of hard queries with 'correct' answers.
Most other queries are inherently subjective. To minimise bias you should ask multiple users to try your search engine and rate the results for comparison with the original. Here is an example of a computer science paper that does something similar:
http://www.cs.uic.edu/~liub/searchEval/SearchEngineEvaluation.htm
Regarding specific comparison of the algorithms, although obvious, what you measure depends on what you're interested in knowing. For example, you can compare efficiency in computation, memory usage, crawling overhead or time to return results. If you are trying to produce very specific behaviour, such as running specialist searches (e.g. a literature search) for certain parameters, then you need to explicitly test this.
Heuristics for relevance are also a useful check. For example, when someone uses search terms that are probably 'programming-related', do you tend to get more results from stackoverflow.com? Would your search results be better if you did? If you are providing a set of trust weightings for specific sites or domains (e.g. rating .edu or .ac.uk domains as more trustworthy for technical results), then you need to test the effectiveness of these weightings.
I don't think you will find a final mathematical solution if that is your goal. In order to rate a given algorithm, you require standards and goals that must be accomplished.
For example, if your goal is to improve the process of page ranking then decide if you are judging the efficiency of the algorithm or the accuracy. Judging efficiency means that you time your code for a consistent large data set and record results. You would then work with your algorithm to improve the time.
If your goal is to improve accuracy then you need to define what is "inaccurate". If you search for "Cup" you can only say that the first site provided is the "best" if you yourself can accurately define what is the best answer for "Cup".
My suggestion for you would be to narrow the scope of your experiment. Define one or two qualities of a search engine that you feel need refinement and work towards improving them.
In the comments you've said "I have heard about a way to measure the quality of the search engines by counting how many time a user need to click a back button before finding the link he wants , but I can use this technique because you need users to test your search engine and that is a headache itself". Well, if you put your engine on the web for free for a few days and advertise a little you will probably get at least a couple dozen tries. Provide these users with the old or new version at random, and measure those clicks.
Other possibility: assume Google is by definition perfect, and compare your answer to its for certain queries. (Maybe sum of distance of your top ten links to their counterparts at Google, for example: if your second link is google's twelveth link, that's 10 distance). That's a huge assumption, but far easier to implement.
First, let me start out by saying, kudos to you for attempting to apply traditional research methods to search engine results. Many SEO's have done this before you, and generally keep this to themselves as sharing "amazing findings" usually means you can't exploit or have the upper hand anymore, this said I will share as best I can some pointers and things to look for.
Different searches execute different algorithms.
Broad Searches
For instance in a broad term search, engines tend to return a variety of results. Common part of these results include
Which of these result types are thrown into the mix can vary based on the word.
Example: Cats returns images of cats, and news, Shoes returns local shopping for shoes. (this is based on my IP in Chicago on October 6th)
The goal in returning results for a broad term is to provide a little bit of everything for everyone so that everyone is happy.
Regional Modifiers
Generally any time a regional term is attached to a search, it will modify the results greatly. If you search for "Chicago web design" because the word Chicago is attached, the results will start with a top 10 regional results. (these are the one liners to the right of the map), after than 10 listings will display in general "result fashion".
The results in the "top ten local" tend to be drastically different than those in organic listing below. This is because the local results (from google maps) rely on entirely different data for ranking.
Example: Having a phone number on your website with the area code of Chicago will help in local results... but NOT in the general results. Same with address, yellow book listing and so forth.
Results Speed
Currently (as of 10/06/09) Google is beta testing "caffeine" The main highlight of this engine build is that it returns results in almost half the time. Although you may not consider Google to be slow now... speeding up an algorithm is important when millions of searches happen every hour.
Reducing Spam Listings
We have all found experienced a search that was riddled with spam. The new release of Google Caffeine http://www2.sandbox.google.com/ is a good example. Over the last 10+ one of the largest battles online has been between Search Engine Optimizers and Search Engines. Gaming google (and other engines) is highly profitable and what Google spends most of its time combating.
A good example is again the new release of Google Caffeine. So far my research and also a few others in the SEO field are finding this to be the first build in over 5 years to put more weight on Onsite elements (such as keywords, internal site linking, etc) than prior builds. Before this, each "release" seemed to favor inbound links more and more... this is the first to take a step back towards "content".
Ways to test an algorythm.
Compare two builds of the same engine. This is currently possible by comparing Caffeine (see link above or google, google caffeine) and the current Google.
Compare local results in different regions. Try finding search terms like web design, that return local results without a local keyword modifier. Then, use a proxy (found via google) to search from various locations. You will want to make sure you know the proxies location (find a site on google that will tell your your IP address geo IP zipcode or city). Then you can see how different regions return different results.
Warning... DONT pick the term locksmith... and be wary of any terms that when returning result, have LOTS of spammy listings.. Google local is fairly easy to spam, especially in competitive markets.
Do as mentioned in a prior answer, compare how many "click backs" users require to find a result. You should know, currently, no major engines use "bounce rates" as indicators of sites accuracy. This is PROBABLY because it would be EASY to make it look like your result has a bounce rate in the 4-8% range without actually having one that low... in other words it would be easy to game.
Track how many search variations users use on average for a given term in order to find the result that is desired. This is a good indicator of how well an engine is smart guessing the query type (as mentioned WAY up in this answer).
**Disclaimer. These views are based on my industry experience as of October 6th, 2009. One thing about SEO and engines is they change EVERY DAY. Google could release Caffeine tomorrow, and this would change a lot... that said, this is the fun of SEO research!
Cheers
Information scientists commonly use precision and recall as two competing measures of quality for an information retrieval system (like a search engine).
So you could measure your search engine's performance relative to Google's by, for example, counting the number of relevant results in the top 10 (call that precision) and the number of important pages for that query that you think should have been in the top 10 but weren't (call that recall).
You'll still need to compare the results from each search engine by hand on some set of queries, but at least you'll have one metric to evaluate them on. And the balance of these two is important too: otherwise you can trivially get perfect precision by not returning any results or perfect recall by returning every page on the web as a result.
The Wikipedia article on precision and recall is quite good (and defines the F-measure which takes into account both).
Embrace the fact that the quality of search results are ultimately subjective. You should have multiple scoring algorithms for your comparison: The old one, the new one, and a few control groups (e.g. scoring by URI length or page size or some similarly intentionally broken concept). Now pick a bunch of queries that exercise your algorithms, say a hundred or so. Let's say you end up with 4 algorithms total. Make a 4x5 table, displaying the first 5 results of a query across each algorithm. (You could do top ten, but the first five are way more important.) Be sure to randomize which algorithm appears in each column. Then plop a human in front of this thing and have them pick which of the 4 result sets they like best. Repeat across your entire query set. Repeat for as many more humans as you can stand. This should give you a fair comparison based on total wins for each algorithm.
Create an app like this that compares and extracts the data. Then run a test with 50 different things you need to look for and then compare with the results you want.
I have had to test a search engine professionally. This is what I did.
The search included fuzzy logic. The user would type into a web page "Kari Trigger", and the search engine would retrieve entries like "Gary Trager", "Trager, C", "Corey Trager", etc, each with a score from 0->100 so that I could rank them from most likely to least likely.
First, I re-architected the code so that it could be executed removed from the web page, in a batch mode using a big file of search queries as input. For each line in the input file, the batch mode would write out the top search result and its score. I harvested thousands of actual search queries from our production system and ran them thru the batch setup in order to establish a baseline.
From then on, each time I modified the search logic, I would run the batch again and then diff the new results against the baseline. I also wrote tools to make it easier to see the interesting parts of the diff. For example, I didn't really care if the old logic returned "Corey Trager" as an 82 and the new logic returned it as an 83, so my tools would filter those out.
I could not have accomplished as much by hand-crafting test cases. I just wouldn't have had the imagination and insight to have created good test data. The real world data was so much richer.
So, to recap:
1) Create a mechanism that lets you diff the results of running new logic versus the results of prior logic.
2) Test with lots of realistic data.
3) Create tools that help you work with the diff, filtering out the noise, enhancing the signal.