1 / 32

Advanced Database Systems F24DS2 / F29AT2

Advanced Database Systems F24DS2 / F29AT2. The Apriori Algorithm. Reading. The main technical material (the Apriori algorithm and its variants) in this lecture is based on: Fast Algorithms for Mining Association Rules, by Rakesh Agrawal and Ramakrishan Sikant, IBM Almaden Research Center

adellac
Télécharger la présentation

Advanced Database Systems F24DS2 / F29AT2

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Advanced Database SystemsF24DS2 / F29AT2 The Apriori Algorithm David Corne, room EM G.39, x 3410, dwcorne@macs.hw.ac.uk / any questions, feel free to contact me

  2. Reading The main technical material (the Apriori algorithm and its variants) in this lecture is based on: Fast Algorithms for Mining Association Rules, by Rakesh Agrawal and Ramakrishan Sikant, IBM Almaden Research Center You don’t have to read that paper, but why not read it anyway – the pdf is on my teaching resources page. David Corne, room EM G.39, x 3410, dwcorne@macs.hw.ac.uk / any questions, feel free to contact me

  3. We will be talking about datasets like that on the following page. This is a transaction database, where each record represents a transaction between (usually) a customer and a shop. Each record in a supermarket’s transaction DB, for example, corresponds to a basket of specific items. David Corne, room EM G.39, x 3410, dwcorne@macs.hw.ac.uk / any questions, feel free to contact me

  4. ID apples, beer, cheese, dates, eggs, fish, glue, honey, ice-cream David Corne, room EM G.39, x 3410, dwcorne@macs.hw.ac.uk / any questions, feel free to contact me

  5. And we will also be talking about things like rules, confidence and coverage (see last lecture). But note that we will now talk about itemsets instead of rules. Also, the coverage of a rule is the same as the support of an items. Don’t get confused! David Corne, room EM G.39, x 3410, dwcorne@macs.hw.ac.uk / any questions, feel free to contact me

  6. Find rules in two stages Agarwal and colleagues divided the problem of finding good rules into two phases: • Find all itemsets with a specified minimal support (coverage).An itemset is just a specific set of items, e.g. {apples, cheese}. The Apriori algorithm can efficiently find all itemsets whose coverage is above a given minimum. • Use these itemsets to help generate interersting rules. Having done stage 1, we have considerably narrowed down the possibilities, and can do reasonably fast processing of the large itemsets to generate candidate rules. David Corne, room EM G.39, x 3410, dwcorne@macs.hw.ac.uk / any questions, feel free to contact me

  7. Terminology k-itemset: a set of k items. E.g. {beer, cheese, eggs} is a 3-itemset {cheese} is a 1-itemset {honey, ice-cream} is a 2-itemset support: an itemset has support s%if s% of the records in the DB contain that itemset. minimum support: the Apriori algorithm starts with the specification of a minimum level of support, and will focus on itemsets with this level or above. David Corne, room EM G.39, x 3410, dwcorne@macs.hw.ac.uk / any questions, feel free to contact me

  8. Terminology large itemset: doesn’t mean an itemset with many items. It means one whose support is at least minimum support. Lk : the set of all large k-itemsets in the DB. Ck : a set of candidate large k-itemsets. In the algorithm we will look at, it generates this set, which contains all the k-itemsets that might be large, and then eventually generates the set above. David Corne, room EM G.39, x 3410, dwcorne@macs.hw.ac.uk / any questions, feel free to contact me

  9. Terminology sets: Let A be a set (A = {cat, dog}) and let B be a set (B = {dog, eel, rat}) and let C = {eel, rat} I use `A + B’ to mean A union B. So A + B = {cat, dog, eel. Rat} When X is a subset of Y, I use Y – X to mean the set of things in Y which are not in X. E.g. B – C = {dog} David Corne, room EM G.39, x 3410, dwcorne@macs.hw.ac.uk / any questions, feel free to contact me

  10. ID a, b, c, d, e, f, g, h, i E.g. 3-itemset {a,b,h} has support 15% 2-itemset {a, i} has support 0% 4-itemset {b, c, d, h} has support 5% If minimum support is 10%, then {b} is a large itemset, but {b, c, d, h} Is a small itemset! David Corne, room EM G.39, x 3410, dwcorne@macs.hw.ac.uk / any questions, feel free to contact me

  11. The Apriori algorithm for finding large itemsets efficiently in big DBs 1: Find all large 1-itemsets 2: For (k = 2 ; while Lk-1 is non-empty; k++) 3 {Ck = apriori-gen(Lk-1) 4 For each c in Ck, initialise c.count to zero 5 For all records r in the DB • {Cr = subset(Ck, r); For each c in Cr , c.count++ } 7 Set Lk:= all c in Ckwhose count >= minsup 8 } /* end -- return all of the Lksets. David Corne, room EM G.39, x 3410, dwcorne@macs.hw.ac.uk / any questions, feel free to contact me

  12. Explaining the Apriori Algorithm … 1: Find all large 1-itemsets To start off, we simply find all of the large 1-itemsets. This is done by a basic scan of the DB. We take each item in turn, and count the number of times that item appears in a basket. In our running example, suppose minimum support was 60%, then the only large 1-itemsets would be: {a}, {b}, {c}, {d} and {f}. So we get L1 = { {a}, {b}, {c}, {d}, {f}} David Corne, room EM G.39, x 3410, dwcorne@macs.hw.ac.uk / any questions, feel free to contact me

  13. Explaining the Apriori Algorithm … 1: Find all large 1-itemsets 2: For (k = 2 ; while Lk-1 is non-empty; k++) We already have L1. This next bit just means that the remainder of the algorithm generates L2, L3 , and so on until we get to an Lk that’s empty. How these are generated is like this: David Corne, room EM G.39, x 3410, dwcorne@macs.hw.ac.uk / any questions, feel free to contact me

  14. Explaining the Apriori Algorithm … 1: Find all large 1-itemsets 2: For (k = 2 ; while Lk-1 is non-empty; k++) 3 {Ck = apriori-gen(Lk-1) Given the large k-1-itemsets, this step generates some candidate k-itemsets that might be large. Because of how apriori-gen works, the set Ckis guaranteed to contain all the large k-itemsets, but also contains some that will turn out not to be `large’. David Corne, room EM G.39, x 3410, dwcorne@macs.hw.ac.uk / any questions, feel free to contact me

  15. Explaining the Apriori Algorithm … 1: Find all large 1-itemsets 2: For (k = 2 ; while Lk-1 is non-empty; k++) 3 {Ck = apriori-gen(Lk-1) 4 For each c in Ck, initialise c.count to zero We are going to work out the support for each of the candidate k-itemsets in Ck, by working out how many times each of these itemsets appears in a record in the DB.– this step starts us off by initialising these counts to zero. David Corne, room EM G.39, x 3410, dwcorne@macs.hw.ac.uk / any questions, feel free to contact me

  16. Explaining the Apriori Algorithm … 1: Find all large 1-itemsets 2: For (k = 2 ; while Lk-1 is non-empty; k++) 3 {Ck = apriori-gen(Lk-1) 4 For each c in Ck, initialise c.count to zero 5 For all records r in the DB • {Cr = subset(Ck, r); For each c in Cr , c.count++ } We now take each record r in the DB and do this: get all the candidate k-itemsets from Ck that are contained in r. For each of these, update its count. David Corne, room EM G.39, x 3410, dwcorne@macs.hw.ac.uk / any questions, feel free to contact me

  17. Explaining the Apriori Algorithm … 1: Find all large 1-itemsets 2: For (k = 2 ; while Lk-1 is non-empty; k++) 3 {Ck = apriori-gen(Lk-1) 4 For each c in Ck, initialise c.count to zero 5For all records r in the DB • {Cr = subset(Ck, r); For each c in Cr , c.count++ } 7 Set Lk:= all c in Ckwhose count >= minsup Now we have the count for every candidate. Those whose count is big enough are valid large itemsets of the right size. We therefore now have Lk, We now go back into the for loop of line 2 and start working towards finding Lk+1 David Corne, room EM G.39, x 3410, dwcorne@macs.hw.ac.uk / any questions, feel free to contact me

  18. Explaining the Apriori Algorithm … 1: Find all large 1-itemsets 2: For (k = 2 ; while Lk-1 is non-empty; k++) 3 {Ck = apriori-gen(Lk-1) 4 For each c in Ck, initialise c.count to zero 5For all records r in the DB • {Cr = subset(Ck, r); For each c in Cr , c.count++ } • Set Lk:= all c in Ckwhose count >= minsup 8 } /* end -- return all of the Lksets. We finish at the point where we get an empty Lk. The algorithm returns all of the (non-empty) Lk sets, which gives us an excellent start in finding interesting rules (although the large itemsets themselves will usually be very interesting and useful. David Corne, room EM G.39, x 3410, dwcorne@macs.hw.ac.uk / any questions, feel free to contact me

  19. apriori-gen : notes Suppose we have worked out that the large 2-itemsets are: L2 = { {milk, noodles}, {milk, tights}, {noodles, quorn}} apriori-gen now generates 3-itemsets that all may be large. An obvious way to do this would be to generate all of the possible 3-itemsets that you can make from {milk, noodles, tights, quorn}. But this would include, for example, {milk, tights, quorn}. Now, if this really was a large 3-itemset, that would mean the number of records containing all three is >= minsup; this means it would have to be true that the number of records containing {tights, quorn} is >= minsup. But, it can’t be, because this is not one of the large 2-itemsets. David Corne, room EM G.39, x 3410, dwcorne@macs.hw.ac.uk / any questions, feel free to contact me

  20. apriori-gen : the join step apriori-gen is clever in generating not too many candidate large itemsets, but making sure to not lose any that do turn out to be large. To explain it, we need to note that there is always an ordering of the items. We will assume alphabetical order, and that the datastructures used always keep members of a set in alphabetical order. a < b will mean that a comes before b in alphabetical order. Suppose we have Lkand wish to generate Ck+1 First we take every distinct pair of sets in Lk {a1, a2 , … ak} and {b1, b2 , … bk}, and do this: in all cases where {a1, a2 , … ak-1} = {b1, b2 , … bk-1}, and ak < bk, {a1, a2 , … ak, bk} is a candidate k+1-itemset. David Corne, room EM G.39, x 3410, dwcorne@macs.hw.ac.uk / any questions, feel free to contact me

  21. An illustration of that Suppose the 2-itemsets are: L2 = { {milk, noodles}, {milk, tights}, {noodles, quorn}, {noodles, peas}, {noodles, tights}} The pairs that satisfy this: {a1, a2 , … ak-1} = {b1, b2 , … bk-1}, and ak < bk, are: {milk, noodles}|{milk, tights} {noodles, peas}|{noodles, quorn} {noodles, peas}|{noodles, tights} {noodles, quorn}|{noodles, tights} So the candidate 3-itemsets are: {milk, noodles, tights}, {noodles, peas, quorn} {noodles, peas, tights}, {noodles, quorn, tights} David Corne, room EM G.39, x 3410, dwcorne@macs.hw.ac.uk / any questions, feel free to contact me

  22. apriori-gen : the prune step Now we have some candidate k+1 itemsets, and are guaranteed to have all of the ones that possibly could be large, but we have the chance to maybe prune out some more before we enter the next stage of Apriori that counts their support. In the prune step, we take the candidate k+1 itemsets we have, and remove any for which some 2-subset of it is not a large k-itemset. Such couldn’t possibly be a large k+1-itemset. E.g. in the current example, we have (n = noodles, etc): L2 = { {milk, n}, {milk, tights}, {n, quorn}, {n, peas}, {n, tights}} And candidate k+1-itemsets so far: {m, n, t}, {n, p, q}, {n, p, t}, {n, q, t} Now, {p, q} is not a 2-itemset, so {n,p,q} is pruned. {p,t} is not a 2-itemset, so {n,p,t} is pruned {q,t} is not a 2-itemset, so {n,q,t} is pruned. After this we finally have C3 = {{milk, noodles, tights}} David Corne, room EM G.39, x 3410, dwcorne@macs.hw.ac.uk / any questions, feel free to contact me

  23. What can you say about the coverage of {apples, milk}? Understanding rules The Apriori algorithm finds interesting (i.e. frequent) itemsets. E.g. it may find that {apples, bananas, milk} has coverage 30% -- so 30% of transactions contain each of these three things. We can invent several potential rules, e.g.: IF basket contains apples and bananas, it also contains MILK. Suppose support of {a, b} is 40%; what is the confidence of this rule? David Corne, room EM G.39, x 3410, dwcorne@macs.hw.ac.uk / any questions, feel free to contact me

  24. Understanding rules II Suppose itemset A = {beer, cheese, eggs} has 30% support in the DB {beer, cheese} has 40%, {beer, eggs} has 30%, {cheese, eggs} has 50%, and each of beer, cheese, and eggs alone has 50% support.. What is the confidence of: IF basket contains Beer and Cheese, THEN basket also contains Eggs ? The confidence of a rule if A then B is simply: support(A + B) / support(A). So it’s 30/40 = 0.75 ; this rule has 75% confidence What is the confidence of: IF basket contains Beer, THEN basket also contains Cheese and Eggs ? 30 / 50 = 0.6 so this rule has 60% confidence David Corne, room EM G.39, x 3410, dwcorne@macs.hw.ac.uk / any questions, feel free to contact me

  25. Understanding rules III If the following rule has confidence c: If A then B and if support(A) = 2 * support(B), what can be said about the confidence of: If B then A confidence c is support(A + B) / support(A) = support(A + B) / 2 * support(B) Let d be the confidence of ``If B then A’’. d is support(A+B / support(B) -- Clearly, d = 2c E.g. A might be milk and B might be newspapers David Corne, room EM G.39, x 3410, dwcorne@macs.hw.ac.uk / any questions, feel free to contact me

  26. What this lecture was about • The Apriori algorithm for efficiently finding frequent large itemsets in large DBs • Associated terminology • Associated notes about rules, and working out the confidence of a rule based on the support of its component itemsets David Corne, room EM G.39, x 3410, dwcorne@macs.hw.ac.uk / any questions, feel free to contact me

  27. Appendix David Corne, room EM G.39, x 3410, dwcorne@macs.hw.ac.uk / any questions, feel free to contact me

  28. ID a, b, c, d, e, f, g A full run through of Apriori We will assume this is our transaction database D and we will assume minsup is 4 (20%) This will not be run through in the lecture; it is here to help with revision David Corne, room EM G.39, x 3410, dwcorne@macs.hw.ac.uk / any questions, feel free to contact me

  29. First we find all the large 1-itemsets. I.e., in this case, all the 1-itemsets that are contained by at least 4 records in the DB. In this example, that’s all of them. So, L1 = {{a}, {b}, {c}, {d}, {e}, {f}, {g}} Now we set k = 2 and run apriori-gen to generate C2 The join step when k=2just gives us the set of all alphabetically ordered pairs from L1, and we cannot prune any away, so we have C2 = {{a, b}, {a, c}, {a, d}, {a, e}, {a, f}, {a, g}, {b, c}, {b, d}, {b, e}, {b, f}, {b, g}, {c, d}, {c, e}, {c, f}, {c, g}, {d, e}, {d, f}, {d, g}, {e, f}, {e, g}, {f, g}} David Corne, room EM G.39, x 3410, dwcorne@macs.hw.ac.uk / any questions, feel free to contact me

  30. So we have C2 = {{a, b}, {a, c}, {a, d}, {a, e}, {a, f}, {a, g}, {b, c}, {b, d}, {b, e}, {b, f}, {b, g}, {c, d}, {c, e}, {c, f}, {c, g}, {d, e}, {d, f}, {d, g}, {e, f}, {e, g}, {f, g}} Line 4 of the Apriori algorithm now tells us set a counter for each of these to 0. Line 5 now prepares us to take each record in the DB in turn, and find which of those in C2 are contained in it. The first record r1 is: {a, b, d, g}. Those of C2 it contains are: {a, b}, {a, d}, {a, g}, {a, d}, {a, g}, {b, d}, {b, g}, {d, g}. Hence Cr1 = {{a, b}, {a, d}, {a, g}, {a, d}, {a, g}, {b, d}, {b, g}, {d, g}} and the rest of line 6 tells us to increment the counters of these itemsets. The second record r2 is:{c, d, e}; Cr2 = {{c, d}, {c, e}, {d, e}}, and we increment the counters for these three itemsets. … After all 20 records, we look at the counters, and in this case we will find that the itemsets with >= minsup (4) counters are: {a, d}, {c, e}. So, L2 = {{a, c}, {a, d}, {c, d}, {c, e}, {c, f}} David Corne, room EM G.39, x 3410, dwcorne@macs.hw.ac.uk / any questions, feel free to contact me

  31. So we have L2 = {{a, c}, {a, d}, {c, d}, {c, e}, {c, f}} We now set k = 3 and run apriori-gen on L2 . The join step finds the following pairs that meet the required pattern: {a, c}:{a, d} {c, d}:{c, e} {c, d}:{c, f} {c, e}:{c, f} This leads to the candidates 3-itemsets: {a, c, d}, {c, d, e}, {c, d, f}, {c, e, f} We prune {c, d, e} since {d, e} is not in L2 We prune {c, d, f} since {d, f} is not in L2 We prune {c, e, f} since {e, f} is not in L2 We are left with C3 = {a, c, d} We now run lines 5—7, to count how many records contain {a, c, d}. The count is 4, so L3 = {a, c, d} David Corne, room EM G.39, x 3410, dwcorne@macs.hw.ac.uk / any questions, feel free to contact me

  32. So we have L3 = {a, c, d} We now set k = 4, but when we run apriori-gen on L3 we get the empty set, and hence eventually we find L4 = {} This means we now finish, and return the set of all of the non-empty Ls – these are all of the large itemsets: Result = {{a}, {b}, {c}, {d}, {e}, {f}, {g}, {a, c}, {a, d}, {c, d}, {c, e}, {c, f}, {a, c, d}} Each large itemset is intrinsically interesting, and may be of business value. Simple rule-generation algorithms can now use the large itemsets as a starting point. David Corne, room EM G.39, x 3410, dwcorne@macs.hw.ac.uk / any questions, feel free to contact me

More Related