PyData - June 6th - Introducing SixFifty

John Sandall on SixFifty's Story: Why Are UK Elections So Hard To Predict?

When Theresa May announced plans on April 18th for the UK to hold a general election it was met with much cynicism. However, as self-confessed psephologists (and huge fans of Nate Silver's FiveThirtyEight datablog), we instead were thrilled at the opportunity. SixFifty is a collaboration of data scientists, software engineers, data journalists and political operatives brought together within hours of the snap general election being announced. Our goals:

• Understand why forecasting elections in the UK using open data is notoriously difficult, and to see how far good statistical practice and modern machine learning methods can take us.

• Make political and demographic data more open and accessible by showcasing and releasing cleaned versions of the datasets we're using.

• We also hope that by communicating our methodology at a non-technical level we will contribute to improving statistical literacy, especially around concepts fundamental to elections and polling.

About the speaker:

John Sandall spends his days working as a self-employed data science consultant and his nights working as a Data Science Instructor at General Assembly. Previously he was Lead Data Scientist at mobile ticketing startup YPlan, quantitative analyst at Apple, strategy/tech lead for non-profit startup STIR Education, co-founder of an ed-tech startup, and dabbled in genomics at Imperial College London.

Transcript

1. BREAK INTO DATA SCIENCE Introducing SixFifty John Sandall 6th June 2017 @john_sandall @SixFiftyData
2. BREAK INTO DATA SCIENCE 18th April 2017
3. WHAT IS DATA SCIENCE? Call to arms
4. WHAT IS DATA SCIENCE? Inspiration
5. WHAT IS DATA SCIENCE? Sidenote: Why am I addicted to poll trackers? Habit Forming 101 1. Cue
6. WHAT IS DATA SCIENCE? Sidenote: Why am I addicted to poll trackers? Habit Forming 101 1. Cue 2. Routine
7. Sidenote: Why am I addicted to poll trackers? Habit Forming 101 1. Cue 2. Routine 3. Reward "Predictable feedback loops don’t create desire" – Nir Eyal
8. The UK has 650 parliamentary constituencies. Why SixFifty?
9. BREAK INTO DATA SCIENCE General Election Forecasts
10. Uniform National Swing: A Case Study Welcome to Sheffield Hallam 2010 results for Sheffield Hallam • CON: 24% • LAB: 16% • LD: 53% MP: Nick Clegg.
11. Step 1. Compare national results with latest polling 2010 national results • CON: 36% • LAB: 29% • LD: 23% 2015 polling • CON: 33% • LAB: 33% • LD: 9% Uniform National Swing: A Case Study
12. Step 2. Calculate "uniform national swing" (i.e. uplift) 2010 -> 2015 UNS • CON: -8% • LAB: +13% • LD: -62% Uniform National Swing: A Case Study
13. Step 3. Apply UNS to each constituency 2015 forecast for Sheffield Hallam • CON: 24% less 8% = 22% • LAB: 16% add 13% = 18% • LD: 53% less 62% = 20% Uniform National Swing: A Case Study
14. Step 4. Forecast winner 2015 forecast for Sheffield Hallam • CON: 22% <- Conservative victory! • LAB: 18% • LD: 20% Uniform National Swing: A Case Study
15. Step 5. So who won? 2015 forecast for Sheffield Hallam • CON: 22% <- Conservative victory! • LAB: 18% • LD: 20% 2015 result for Sheffield Hallam • CON: 14% • LAB: 36% • LD: 40% <- Lib Dem victory! Uniform National Swing: A Case Study
16. Is there a better way? • Use rigorous & modern modelling techniques. • Cross-validate, backtest, evaluate for predictive accuracy. • Open source our code, data, methodology. • Blend in multiple data sources, not just polling. • Greater understanding of what drives election outcomes. Uniform National Swing: A Case Study
17. BREAK INTO DATA SCIENCE Polling
18. Average forecast (left) vs actual results (right) for the 2015 general election What went wrong in 2015?
19. • "Shy Tories"? • "Lazy Labour"? • Biased sampling? • Herding? What went wrong in 2015?
20. • "Shy Tories"? Mostly a convenient myth. • "Lazy Labour"? • Biased sampling? • Herding? What went wrong in 2015?
21. • "Shy Tories"? Mostly a convenient myth. • "Lazy Labour"? Partially true. • Biased sampling? • Herding? What went wrong in 2015?
22. • "Shy Tories"? Mostly a convenient myth. • "Lazy Labour"? Partially true. • Biased sampling? Definitely true. • Herding? What went wrong in 2015?
23. What went wrong in 2015?
24. • "Shy Tories"? Mostly a convenient myth. • "Lazy Labour"? Partially true. • Biased sampling? Definitely true. • Herding? Definitely true. What went wrong in 2015?
25. • "Shy Tories"? Mostly a convenient myth. • "Lazy Labour"? Partially true. • Biased sampling? Definitely true. • Herding? Definitely true. One survey found that 75% of US adults don't trust surveys. What went wrong in 2015?
26. BREAK INTO DATA SCIENCE Polling data
27. What does polling data look like?
28. Raw data contains: • Voting Intention (“Which party will you be voting for on June 8th?”) • Party leader satisfaction • Policy preferences (“Do you think tuition fees should be abolished?”) • Demographic background (location, gender, age, education, etc) • Voted during EU Referendum? Remain or Leave? • Voted during 2015 general election? Which party voted for? • Questions designed to gauge likelihood of voting
29. Raw data looks like:
30. Automated data extraction? http://tabula.technology/
31. Open polling data? bit.ly/UKPoliticsDatasets
32. Open polling data? http://opinionbee.uk/
33. Easily accessible? Methodology? Regional?
34. Rolling our own open polling data pipeline 1. Alert
35. Rolling our own open polling data pipeline 1. Alert 2. Update
36. Rolling our own open polling data pipeline 1. Alert 2. Update 3. Automate https://github.com/six50/pipeline/
37. Regional polling data
38. Regional polling data
39. Regional polling data
40. D3 poll tracker https://sixfifty.org.uk/polls
41. Polling data on SixFifty.org.uk https://sixfifty.org.uk/polls
42. BREAK INTO DATA SCIENCE Forecast Model
43. The Plan 1. Replicate UNS model 2. Check predictions against other forecasts 3. Evaluate with historical election data 4. Build ML model 5. Iterate...
44. UNS models National UNS forecast for 2017 • CON: 337 • LAB : 236 • SNP : 47 • LD : 6 • PC : 5 • Other: 19
45. National UNS forecast for 2017 • CON: 337 • LAB : 236 • SNP : 47 • LD : 6 • PC : 5 • Other: 19 Regional UNS forecast for 2017 • CON: 342 • LAB : 236 • SNP : 43 • LD : 6 • PC : 4 • Other: 19 UNS models
46. UNS models
47. Model Evaluation • Repeat for GE2010 -> GE2015 • Forecast GE2015 seat-by-seat • Evaluate: • National Vote Share (MAE) = 2% error • National Seat Count (total) = 135 incorrectly called • Seat-by-seat Accuracy = 79% correctly called • Seat vote share MAE = 4.6% mean error / party / seat
48. Build ML Model • Polling data only for 2010, 2015, 2017 • CON / LAB / LD / UKIP / GRN only • Calculate and forecast using UNS => useful feature! • Features: • Region • Electorate (registered, total who voted, total votes cast) • Party • Previous election: total votes, vote share, won constituency? • Current election: poll vote share, swing • UNS forecast: vote share (%), predicted winner
49. Build ML Model
50. Build ML Model • Evaluation • 5x5 cross-validation on GE2015 predictions from GE2010 data • Models (scikit-learn) • Linear Regression (Simple, Lasso, Ridge) • Ensemble (Random Forest, Gradient Boosting, Extra Trees) • Neural net (MLPRegressor) • Tune best default model (Gradient Boosting)
51. Build ML Model • Tune best default model (Gradient Boosting)
52. • Seat vote share MAE • UNS model = 4.6% mean error / party / seat • GB model = 2.3% mean error / party / seat Evaluate ML Model
53. Feature Importance
54. Regional UNS forecast for 2017 • CON: 342 • LAB : 236 • SNP : 43 • LD : 6 • PC : 4 • Other: 19 ML forecast for 2017 • CON: 332 • LAB : 232 • SNP : 56 • LD : 5 • PC : 4 • Other: 19 Final Prediction
55. •Very friendly to SNP! •Overfitting to 2015 election •Wish we had more data! Caveats
56. BREAK INTO DATA SCIENCE Next steps
57. BREAK INTO DATA SCIENCE Thank You
58. BREAK INTO DATA SCIENCE UPDATE This section has been added after the PyData talk. 1. Final forecast. 2. Comparison with election result. 3. Comparison with other forecasts.
59. Regional UNS forecast for 2017 • CON: 342 • LAB : 236 • SNP : 43 • LD : 6 • PC : 4 • GRN: 1 • Other: 19 ML forecast for 2017 • CON: 336 • LAB : 232 • SNP : 52 • LD : 6 • PC : 4 • GRN: 1 • Other: 19 Final Prediction ← Majority of 34 ← Majority of 22
61. ML forecast for 2017 • CON: 336 • LAB : 232 • SNP : 52 • LD : 6 • PC : 4 • GRN: 1 • Other: 19 Final Prediction ← Majority of 22 Actual result • CON: 317 • LAB : 262 • SNP : 35 • LD : 12 • PC : 4 • GRN: 1 • Other: 19 ← 9 short of majority
63. TL;DR In seven weeks we built the most accurate election forecast built purely from public data.