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Example: Build an “Adult Web Site” Classifier

Crowdsourcing using Mechanical Turk Quality Management and Scalability Panos Ipeirotis – New York University. Example: Build an “Adult Web Site” Classifier. Need a large number of hand-labeled sites Get people to look at sites and classify them as:

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Example: Build an “Adult Web Site” Classifier

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  1. Crowdsourcing using Mechanical TurkQuality Management and Scalability Panos Ipeirotis – New York University

  2. Example: Build an “Adult Web Site” Classifier • Need a large number of hand-labeled sites • Get people to look at sites and classify them as: G (general audience)PG (parental guidance) R(restricted)X (porn) • Cost/Speed Statistics • Undergrad intern: 200 websites/hr, cost: $15/hr

  3. Example: Build an “Adult Web Site” Classifier • Need a large number of hand-labeled sites • Get people to look at sites and classify them as: G (general audience)PG (parental guidance) R(restricted)X (porn) • Cost/Speed Statistics • Undergrad intern: 200 websites/hr, cost: $15/hr • MTurk: 2500 websites/hr, cost: $12/hr

  4. Bad news: Spammers! • Worker ATAMRO447HWJQ • labeled X (porn) sites as G (general audience)

  5. Improve Data Quality through Repeated Labeling • Get multiple, redundant labels using multiple workers • Pick the correct label based on majority vote 11 workers 93% correct 1 worker 70% correct • Probability of correctness increases with numberof workers • Probability of correctness increases with quality of workers

  6. Using redundant votes, we can infer worker quality • Look at our spammer friend ATAMRO447HWJQ together with other 9 workers • We can compute error rates for each worker • Error rates for ATAMRO447HWJQ • P[X → X]=0.847% P[X → G]=99.153% • P[G → X]=0.053% P[G → G]=99.947% Our “friend” ATAMRO447HWJQmainly marked sites as G.Obviously a spammer…

  7. Rejecting spammers and Benefits Random answers error rate = 50% Average error rate for ATAMRO447HWJQ: 49.6% • P[X → X]=0.847% P[X → G]=99.153% • P[G → X]=0.053% P[G → G]=99.947% Action: REJECT and BLOCK Results: • Over time you block all spammers • Spammers learn to avoid your HITS • You can decrease redundancy, as quality of workers is higher

  8. Too much theory? Demo and Open source implementation available at: http://qmturk.appspot.com • Input: • Labels from Mechanical Turk • Some “gold” data (optional) • Cost of incorrect labelings (e.g., XG costlier than GX) • Output: • Corrected labels • Worker error rates • Ranking of workers according to their quality

  9. How to handle free-form answers? • Q: “My task does not have discrete answers….” • A: Break into two HITs: • “Create” HIT • “Vote” HIT • Vote HIT controls quality of Creation HIT • Redundancy controls quality of Voting HIT • Catch: If “creation” very good, in voting workers just vote “yes” • Solution: Add some random noise (e.g. misspell) Creation HIT (e.g. transcribe caption) Voting HIT: Correct or not? Example: Collect URLs

  10. But my free-form is not just right or wrong… Describe this • “Create” HIT • “Improve” HIT • “Compare” HIT Creation HIT (e.g. describe the image) Compare HIT (voting) Which is better? Improve HIT (e.g. improve description) TurkIt toolkit: http://groups.csail.mit.edu/uid/turkit/

  11. version 1: A parial view of a pocket calculator together with some coins and a pen. version 2: A view of personal items a calculator, and some gold and copper coins, and a round tip pen, these are all pocketand wallet sized item used for business, writting, calculating prices or solving math problems and purchasing items. version 3: A close-up photograph of the following items: A CASIO multi-function calculator. A ball point pen, uncapped. Various coins, apparently European, both copper and gold. Seems to be a theme illustration for a brochure or document cover treating finance, probably personal finance. version 4: …Various British coins; two of £1 value, three of 20p value and one of 1p value. … version 8: “A close-up photograph of the following items: A CASIO multi-function, solar powered scientific calculator. A blue ball point pen with a blue rubber grip and the tip extended. Six British coins; two of £1 value, three of 20p value and one of 1p value. Seems to be a theme illustration for a brochure or document cover treating finance - probably personal finance."

  12. Future: Break big task to simple ones and build workflow • Running experiment: Crowdsource big tasks (e.g., tourist guide) My Boss is a Robot (mybossisarobot.com) Nikki Kittur (Carnegie Mellon) + Jim Giles (New Scientist) • Identify sights worth checking out (one tip per worker) • Vote and rank • Brief tips for each monument (one tip per worker) • Vote and rank • Aggregate tips in meaningful summary • Iterate to improve…

  13. Thank you!Questions? “A Computer Scientist in a Business School” http://behind-the-enemy-lines.blogspot.com/ Email: panos@nyu.edu

  14. Correcting biases • Classifying sites as G, PG, R, X • Sometimes workers are careful but biased • Classifies G → P and P → R • Average error rate : too high • Error Rates for CEO of company detecting offensive content (and parent) • P[G → G]=20.0% P[G → P]=80.0% P[G → R]=0.0% P[G → X]=0.0% • P[P → G]=0.0% P[P → P]=0.0%P[P → R]=100.0% P[P → X]=0.0% • P[R → G]=0.0% P[R → P]=0.0% P[R → R]=100.0% P[R → X]=0.0% • P[X → G]=0.0% P[X → P]=0.0% P[X → R]=0.0% P[X → X]=100.0% Is she a spammer?

  15. Correcting biases • Error Rates for Worker: ATLJIK76YH1TF • P[G → G]=20.0% P[G → P]=80.0% P[G → R]=0.0% P[G → X]=0.0% • P[P → G]=0.0% P[P → P]=0.0%P[P → R]=100.0% P[P → X]=0.0% • P[R → G]=0.0% P[R → P]=0.0% P[R → R]=100.0% P[R → X]=0.0% • P[X → G]=0.0% P[X → P]=0.0% P[X → R]=0.0% P[X → X]=100.0% • For ATLJIK76YH1TF, we simply need to “reverse the errors” (technical details omitted) and separate error and bias • True error-rate ~ 9%

  16. Scaling Crowdsourcing: Use Machine Learning • Human labor is expensive, even when paying cents • Need to scale crowdsourcing • Basic idea: Build a machine learning model and use it instead of humans Data from existing crowdsourced answers New Case Automatic Model(through machine learning) Automatic Answer

  17. Tradeoffs for Automatic Models: Effect of Noise Get more data  Improve model accuracy Improve data quality  Improve classification Example Case: Porn or not? Data Quality = 100% Data Quality = 80% Data Quality = 60% Data Quality = 50% 22

  18. Scaling Crowdsourcing: Iterative training • Use machine when confident, humans otherwise • Retrain with new human input → improve model → reduce need for humans Automatic Answer Confident New Case Automatic Model(through machine learning) Not confident Get human(s) to answer Data from existing crowdsourced answers

  19. Tradeoffs for Automatic Models: Effect of Noise Get more data  Improve model accuracy Improve data quality  Improve classification Example Case: Porn or not? Data Quality = 100% Data Quality = 80% Data Quality = 60% Data Quality = 50% 24

  20. Scaling Crowdsourcing: Iterative training, with noise • Use machine when confident, humans otherwise • Ask as many humans as necessary to ensure quality Automatic Answer Confident New Case Automatic Model(through machine learning) Not confident for quality? Not confident Data from existing crowdsourced answers Get human(s) to answer Confident for quality?

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