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Want to Merge Millions of Legal Opinions? It Won't Be Easy.

Michael Lissner

Note: This is the third in the series of posts explaining the work that we did to release the data donation from Lawbox LLC. This is a very technical post exploring and documenting the process we use for extracting meta data and merging it with our current collection. If you're not technically-inclined (or at least curious), you may want to scoot along.

Working with legal data is hard. We all know that, but this post serves to document the many reasons why that's the case and then delves deeply into the ways we dealt with the problems we encountered while importing the Lawbox donation. The data we received from Lawbox contains about 1.6M HTML files and we've spent the past several months working with them to extract good meta data and then merge it with our current corpus. This post is a long and technical one and below I've broken it into two sections explaining this process: Extraction and Merging.


Extraction is a difficult process when working with legal data because it's inevitably quite dirty: Terms aren't used consistently, there are no reliable identifiers, formats vary across jurisdictions, and the data was made by humans, with typos galore. To overcome these issues we use a number of approaches ranging from hundreds of regular expressions to clever heuristics.

The first step we take is to convert the HTML files into an in-memory tree that we can traverse and that we can query using XPath, a variable that contains only the text of the opinion (for later analysis), and a variable that contains simplified versions of the HTML with any headers or other junk stripped out.

From there, the tree, text and simplified tree get sent into various functions that extract the following pieces of meta data:

  • Citations
  • Case name
  • Case date
  • Docket number
  • Judge(s)
  • Jurisdiction

Of these, jurisdiction and citations are by far the hardest. The others are fairly straightforward, though dates are often missing and must be laboriously looked up.

Citations are extracted using our standard citation finder. We've described how it works in the past (pdf), but the basic idea is to tokenize text into valid words and then find valid reporters within the tokens. Whenever a valid reporter is found, you then inch backwards and forwards from it, identifying the volume, page number, year, and any other related information.

Finding the jurisdiction relies on a collection of about 500 regular expressions, each designed to find a specific court. Since the data provided by Lawbox is rather dirty, you can see that these regular expressions do a lot work. Unfortunately this approach isn't enough for many jurisdictions, and for the hard ones we go a step further.

If the regular expressions fail, the next step we take is to use the citation information as a clue towards the jurisdiction. In many cases it works! It's often enough to know that a case is in the California Appellate Reporter or the U.S. reporter. Using that information alone, we can often figure out the hard cases.

But sometimes they're really hard to figure out.

The really hard cases in the Lawbox collection describe their jurisdiction like so: "United States District Court, D. Alabama". Doesn't look hard, but, well, Alabama currently has three district courts, the Middle, Northern and Southern, but it doesn't have a generic "D. Alabama" (at least not since 1824). For the rare case like this, we developed a clever solution: We use the judge information in the case to determine the jurisdiction. Since most judges don't move too much between courts, before we began importing anything we extracted all the judges and made tallies of where they worked. Then, when we encountered a case like the above, we said, "OK, who's the judge in this case, and where does he work?" In almost every case this worked very well, but in some cases it didn't and for those, we simply put the information in manually.

For the remainder of the meta data fields listed, we employed similar tricks, but these were the hardest examples. For the remainder of our approach, you can inspect the code itself. Just be careful of hairballs.


Once all of the meta data is properly extracted, the next step is to merge it with our existing corpus, identifying duplicates and merging them, or simply adding new cases if no duplicate was found.

The merging process takes one of three main avenues:

  • Cases for which there cannot be a duplicate.
  • Cases for which there is exactly one duplicate.
  • Cases for which there are multiple duplicates.

For the vast majority of the Lawbox donation, we were able to simply add the case to our collection without further ado. We determined this by comparing the date and jurisdiction of the new opinion to our collection and seeing if we had any cases from that jurisdiction during that time. If there weren't any cases from that place and time, bingo, the new case couldn't be a duplicate and we could add it straightaway.

For the opinions that might have duplicates, we developed a duplicate-detecting algorithm. The process for this algorithm is as follows.

1: Create a set of candidate documents that might be duplicates by searching our existing corpus. First search it for cases in the same jurisdiction within 15 days of when it was issued and which have the same words in their name. Since names can vary greatly, the last word of the plaintiff and the first word in the defendant are used as queries, but only if those words:

  • aren't uppercase and less than three letters long (indicating an abbreviation);
  • aren't words that occurs very frequently (indicating a stop word);
  • don't contain punctuation or numbers (indicating something out of the norm); and
  • aren't less than two letters long (indicating they're an abbreviation).

Once this query returns, if it has results, we continue to step 2, but if not, we try a new query using the docket number instead of the case name. This often works, but if it fails we try one final time using the citations. Unfortunately we can't use the citations for all queries because prior to this donation we did not have a good collection of citation information.

2: Once we have some cases that our new one might be a duplicate of, we attempt to match up the duplicates by docket number. This often works, but if it doesn't, we gather statistics about the items our new document might be a duplicate of. Specifically, we gather:

  • The edit distance between our new case and each of the candidates;
  • The edit distance between the text of our case and each of the candidates;
  • The difference in length between our case and each of the candidates; and
  • The cosine similarity between our case and each of the candidates.

Once that's gathered, we set it aside and move on to step three.

3: At this point, we compare the case names to see if any of them are good matches. We assume that if we have one candidate, if it has all the same words in its case name as does our new document, and if they're in the right order, it must be a duplicate. So for example, Lissner v. Carver is a duplicate of Michael Lissner v. Brian Carver, but not of Carver v. Lissner (right words but wrong order).

4: If this approach fails, our next step is to attempt a similarity test based on the docket number instead of the case name. This often works, but when it doesn't, we have another approach, using the statistics generated above.

5: Our last approach is the statistical approach. Of the statistics generated above, the cosine similarity is very accurate and the others seem flawed in various ways. Cosine similarity takes all of the words in each case, counts up how many times each one occurs, then plots all of the words in a multi-dimensional vector space. Once we have a vector space for each case, we determine the difference between the new case and each candidate. If the two cases are very similar, they get a high similarity rating. If not, they get a low one. In our experience a good duplicate has a similarity of about 98%, and a dissent to the same case has usually has a slightly lower similarity, generally around 97%. Anything below 90% is unrelated. The extent to which this approach works is remarkable, but it is slow and can lose accuracy if cases have additional words, say, as part of the header information.

6: After all this is done, if we still haven't determined if we have any duplicates, our final approach is to send the new case and all its candidates to a human reviewer, who looks at their contents and makes a determination. Fortunately, this only happens rarely.

The Final Step

Once the meta data is extracted and any duplicates have been found, we take the best parts of each document, merge them together and save it to the index. Once this is complete the document shows up in search, in our bulk files and everywhere else.

This approach took many month's development, and it will receive another round of polish the next time we add a batch of cases. Until then, we hope this post has been educational, and that it can serve as a reference for any data merging projects.

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