I read Jacob Neusner's "The Synoptic Problem in Rabbinic Literature", which appeared in the September 1, 1986 Journal of Biblical Literature.
What is the synoptic problem? In New Testament studies, there is a prominent view that Matthew and Luke used Mark as a source. But there are sayings that Matthew and Luke have in common, which are not in Mark. According to a number of scholars, Matthew and Luke drew these sayings from a Q source. Some believe that there was a Q community. And then there are scholars who think that there were various levels of Q: Q1, Q2, etc. One level was supposedly apocalyptic, but another level was not.
In rabbinic literature, common sayings or variations of a saying show up in different writings, and the question is this: Did all of these documents draw from a common source, a Q, if you will? And Neusner's answer is that they did not. Here are examples that he discusses:
The Tosefta expands upon the Mishnah, so that is why the Mishnah and the Tosefta have material in common---it's not because they're both drawing from a common source. The Sifra tries to establish the authority of Scripture by showing that the Mishnah's rules are based on Scripture, or by demonstrating that Scripture contradicts logic and so we should go with Scripture rather than relying on logical argumentation (which is significant because the Mishnah often doesn't rely on Scripture). But Sifra uses the Mishnah, and so their commonalities are not due to a Q source. According to Neusner, Leviticus Rabbah doesn't even share much material with other sources, so it wasn't intent on drawing from a Q document.
Finally, Neusner argues that, when we look at the sayings that different documents have in common, we do not see a pattern, or salient characteristics that mark them as part of a common source. They are various. Some of them look indigenous to particular sources, such as Genesis Rabbah or Leviticus Rabbah. For Neusner, there was no rabbinic Q source. All we really know is that sayings appear in different places.