Thks a lot: your propositions are OK !

Now, I'd like to extract from the previous set (logs with ratings up to 3), the logs attached to a user who has performed at least 10 ratings (I will compute accuracy and recall and wish to work with user with at least 10 rates). In others words, I retain userid only if the userid appears at least 10 times.

I can select this list of userid:

In[196]:= TableUserIDAuMoins10 = Select[Counts[CorpusReferenceu1test[[All, 1]]], # > 9 &]

Out[196]= <|1 -> 79, 2 -> 12, 5 -> 25, 6 -> 64, 7 -> 135, 8 -> 22, ...|>

In[197]:= Keys[TableUserIDAuMoins10]

Out[197]= {1, 2, 5, 6, 7, 8, 10, 11, 12, 13, ...}

But now, I don't see how I can write "select logs form CorpusReferenceu1test where userid is within Key[TAbleUSerIDAumoins10]... (logs are (userid, itemid, rates), so that Keys[TableUserIDAuMoins10] can be seen as the list of the available userid )

BR