Update: a number have people have pointed out some small errors and some additional APIs that I should look at. See my half-hearted followup: Entity Extraction APIs, once again.

As part of a project I’m working on (more on that later), I wanted to be able to take some text (probably in the form of a web page) and get a list of the important entities/keywords/phrases.

It turns out that there are actually quite a few companies that offer a service like this available freely (at least somewhat) through an API, so I set out to try them all out and assess their quality and suitability for my project. Most of these APIs are provided by companies that do various things in the NLP (natural language processing) realm and/or work with large semantic datasets. Many of the APIs provide a variety of information, only some of which is the set of entities that I’m looking for, so they may have good features that are excluded from my narrow comparison.

Using the APIs

To evaluate the APIs I wrote a script to make use of each one (scroll to the bottom to see it in action). They were fairly similar but the code to handle each one is slightly different. In many cases they offered multiple response formats but I opted for XML for each of them which made things simple enough, and I got used to using SimpleXML in PHP. The main difference between them all is simply the XPath expression needed to pick out the entities. For each API I grab the entity, any available synonyms (minus some de-duplication), and the self-reported relevance score for that entity, if available. If not already sorted by that relevance, I sort them.

An additional issue was that although most of the APIs accepted a URL as input, some required the actual content, in either HTML or plain text. When accepting content from a web page, the service needs to be smart about ignoring web navigation, ads, etc. when determining what is important, and they vary in ability to do that. Alchemy (one of the APIs tested here) also has a web page cleaning API which can be accessed on its own. Results from the Yahoo API were of such low quality that I actually ran the input web page through the web page cleaning API before sending it to Yahoo, and it is those results which are evaluated here.

Most of the analysis here is based off a sample of eight web pages including Wikipedia articles, news articles, and other pages with a lot of text content from a variety of subjects. I have not yet done any analysis of how the quality of the response for each API is affected by the length of the input document.

The APIs

The APIs I tested were, roughly in order of increasing quality,

Comment on this post to let me know if I’m missing any.

API terms

Most APIs today have limits on both how much they can be called, and what you can use them for. Here they are ranked roughly by “most usable terms and limits” to least:

Evri Currently no API limit; essentially no requirements AlchemyAPI 30,000 calls per day (although more may be available); commercial use is definitely okay OpenCalais 50,000 calls per day, 4 calls per second; one must display their logo as-is; if you are syndicating the data it needs to preserve their GUIDs; see details BeliefNetworks 2,000 calls per day, 1 call per second; essentially no requirements Yahoo 5,000 calls per IP per day; non-commercial use only OpenAmplify 1,000 “transactions” per day; note that one call is 1-4 transactions depending on the input type and whether you want all or a subset of the output; commercial use is definitely okay

Please note the standard I-am-not-a-lawyer and that this is just a summary. Please read the terms of service yourself.

Languages

Although my project is English-only for now, ideally there would be support for other languages. All the samples I used were in English so that is what is being used to evaluate the quality, but here is the full list of what languages each API claims to handle:

Yahoo English OpenCalais English, French, Spanish BeliefNetworks A white paper on their website claims that their technology can support multiple languages, but most likely only English is currently supported OpenAmplify English AlchemyAPI English, French, German, Italian, Portuguese, Russian, Spanish, Swedish, however some important features are only available for English Evri English

I have not done any research into similar APIs which are not available for English at all, but if you find one, please let me know and I’ll make a note of it here.

The response from OpenAmplify and AlchemyAPI will also include the language of the input document. For AlchemyAPI this includes 97 languages, not just the ones that the API can handle. If you’re just looking for good language identification, there are other resources for that, some open source. My ancient Language Identification page still has some useful links there.

Number of Entities and Relevance Scores

For the purposes of my project, I see the APIs which return more entities from the document to be more useful, all else being equal. BeliefNetworks allows you to specify the number of entities returned (supposedly up to 75 but it actually returns 76), and as such always returns that number, which is almost always more than any other API. Yahoo returns up to 20 entities (which isn’t documented), which is often the least of any API. Here I list the APIs sorted by number of entities returned from most to least:

BeliefNetworks AlchemyAPI OpenAmplify OpenCalais Yahoo Evri

There are a couple of important caveats here, however. This is based off of a very small sample, so other than BeliefNetworks returning the most, the list could be off. Beyond that, OpenCalais has a fairly small limit on text length (100,000 characters, presumably including the HTML tags) and if the input is too long, it returns no entities at all, just an error message. The ranking above excludes those examples. OpenAmplify has a limit of 2.5K, however they just truncate the document instead of failing (although this counts as an additional “transaction”). Oddly, Evri returned an error of “rejected by content filter” for this news article and returned no entities. Evri’s ranking in the list above is unchanged with or without the inclusion of that example.

Relevance Scores

All of the APIs, with the exception of Yahoo’s, include some metric with each entity rating its relevance to the input document. This is important as every user of any of these APIs would most likely want to establish a minimum relevance threshold for actually making use of the entities. The number of entities comparison above is based on no threshold at all; obviously changing the threshold would affect the comparison. AlchemyAPI and OpenCalais use scores from zero-to-one, however Evri, OpenAmplify, and BeliefNetworks have their own scale. I haven’t yet done any work to normalize all these scores and I think that most likely the best practice would be to independently determine your own threshold on a per-api basis depending on your own needs.

Semantic Links

By semantic links I simply mean that the entities returned have some sort of links or references to additional information about those entities. Although not necessarily required for my project, this may be very useful. Two of the APIs, Evri and AlchemyAPI include this information when they successfully map a found entity to an entity in their own database. Evri provides a reference to the entity in Evri’s own system, whereas AlchemyAPI links to a variety of other sources: the website for the entity, Freebase, MusicBrainz, and others.

In addition to or instead of these semantic links, Evri, AlchemyAPI, and OpenCalais have their own systems of classification and label entities with things like “Person” and “Religion”. See Evri’s most popular ‘facets’, AlchemyAPI Entity Types, and OpenCalais: Metadata Element Simple Output Value for specifics of each. OpenAmplify is even more basic but provides broad categories such as “Locations” and “Proper Nouns”, and entities may be listed in more than one of these broad categories. Yahoo and BeliefNetworks provide no additional context.

Additional Information

Some of these APIs provide a wealth of information that I disregard entirely but could be useful to others. For example Amplify returns a lot of information about the sentiment being expressed about each entity, information about the person who authored the document (e.g. gender, education), the style of the document (e.g. slang usage), and actions expressed within the document. OpenCalais also extracts events and facts from a document, as well as other details per entity such as the ticker symbol for entities which are public companies. AlchemyAPI can extract quotations from the document. Note that this is a summary and not a complete list of all the data that these APIs return.

Synonyms vs. Duplicates

The better APIs here, at least as far as I’ll be using them, succeed at recognizing that “Smith” referred to throughout an article is the same as “John Smith” mentioned in the first sentence. I want duplicates minimized, and for each entity to have as many valid names/synonyms as possible. The APIs differ here significantly.

Evri is definitely the best, followed by AlchemyAPI. Unfortunately AlchemyAPI sometimes misdisambiguates (ooh, no results on Google or Bing for that word yet) which results in incorrect synonyms, however that isn’t a huge problem for me. An example is the article I referred to earlier where AlchemyAPI confuses a Canadian military unit for the British monarch it was named after. Yahoo and OpenCalais fall into the middle. OpenAmplify and BeliefNetworks have a fair number of unmerged duplicate entities. For my purposes, I don’t care if the synonyms come from the input document or an external database, which is what Evri and AlchemyAPI probably use.

Taking a look at each API

Yahoo

This was the only API that I was aware of until recently, and I’ve blogged about it before. The input format is plain text, so since I’m using URLs as input, I have to first extract the text, strip the HTML, and send that. As I mentioned above, the quality was so poor when using web pages as input that the text must first be scrubbed of web page navigation, etc., and I used AlchemyAPI to do that. Even then, the quality was still poor and the API returned things that I would describe more as long phrases than as entities. Given that, not to mention the maximum of 20 entities, and the non-commercial restriction, I don’t see myself making use of this API.

OpenCalais

This API also accepted content rather than a URL. The content format must be specified in the API call. I simply retrieved the URL, and passed all of its content (with HTML) on to OpenCalais. They suggest making sure to remove web page navigation, but without me doing this, that didn’t present a problem. What was a problem was the short maximum document length. To actually use OpenCalais you should make sure to truncate documents before making the request, which is work that I haven’t yet implemented myself. Even when results were returned, the overall quality was mediocre.

The default output format is RDF, which is very verbose and includes a lot more information than I needed. I opted for the Text/Simple format which is actually XML.

For free API users sometimes the response comes back as “server busy” and I experienced this myself sometimes while trying it out.

BeliefNetworks

This API has a simple output, and is somewhat unusual in its functionality. Unlike all the other APIs, where the entities are extracted from the input document, with BeliefNetworks it seems they find entities which are related to the document but not necessarily actually in it. This produces some interesting results that are sometimes good but overall less related than I’d expect, and in one of my examples, completely unrelated and bizarre. Given that, and the frequency of duplicate entities, as mentioned, I would describe the overall quality as mediocre, although usually better than OpenCalais.

OpenAmplify

The most notable feature of OpenAmplify is all the additional information they provide, as described above.

They take input either as a URL or the content itself, and “charge” an additional transaction if you call them using a URL. I used URL input but also tried submitting the HTML, submitting the text (HTML stripped), and submitting the text after putting through the AlchemyAPI web page cleaning, and in all cases the results were about the same or worse.

OpenAmplify notes that they may not be able to follow all URL redirects (although I didn’t test this with any of the APIs), but this issue can be avoided by following the redirects yourself before making the request. As mentioned earlier, they only look at the first 2.5K of input. They also accept RSS/Atom as input, which is a nice feature.

Although I’ve set up my script to remove duplicates it currently misses removing some duplicate entities from OpenAmplify as the entity may be listed several times in the response but with different relevance scores.

One problem I found was that the entities returned usually consisted of a single token (one word) which just made them less useful. Overall, the quality was okay, generally better than BeliefNetworks.

AlchemyAPI

Other than the occasional misdisambiguation, AlchemyAPI is quite good.

Evri

Evri’s API is also quite good, with the biggest flaw being that it doesn’t return very many entities.

Overall Quality Summary

Overall, Evri and AlchemyAPI were definitely the best and most suited for my purposes. The quality of Evri’s was the best across the small sample, although not in all instances, and it didn’t return as many entities as AlchemyAPI. Interestingly these two APIs are also the two which include semantic links and have the least restrictions and high API limits.

OpenAmplify and BeliefNetworks are the runners up. OpenCalais fared poorly in my evaluation, but I suspect it would do better when looking at all the rest that their API. Yahoo’s API unfortunately just wasn’t good enough to use when any of the other APIs are available.

I’m convinced that trying to build a similar service myself is not worth it at all. One thing that I haven’t tried yet is combining these APIs together in some way, although that could potentially improve the results quite a bit.