In data science, sometimes you stumble across an intriguing property in the data. I will tell you a story of a mysterious correlation - from StackOverflow survey it seems that developers who use spaces have higher salaries than those who use tabs. Correlation doesn’t mean causation: using spaces won’t suddenly increase your salary. But what does it all mean? Follow me into a detective investigation that will show you how to approach complex data science problems. I will show you some of the perils of correlation, model fitting and biases - how they can be dangerous and how to deal with them.

2. THE MYSTERIOUS
CORRELATION
A detective story
Evelina Gaba ová
@EVELGAB

8. CORRELATION IS NOT
CAUSATION!

9. CORRELATION AND
CAUSATION

10. EVERYONE LOVES A
NICE CORRELATION!

16. CAUSAL
LANGUAGE
FOR
CORRELATION
DATA

17. THINGS TO CHECK
correlation or causation (randomised study?)
small sample size
result by chance
hidden cause (latent variable)

18. DO STORKS DELIVER
BABIES?
Source: Claude Covo-Farchi

19. Testing the hypothesis
Matthews, R. (2000), Storks Deliver Babies (p= 0.008). Teaching Statistics, 22: 36–38.

22. Storks and country area

23. Birth rate and country area

26. Predicting salary with linear regression
Country
Years of programming experience
Tabs and spaces usage
Developer type and language
Level of formal education (e.g. bachelor’s, master’s,
doctorate)
Whether they contribute to open source
Whether they program as a hobby
Company size

27. WHAT HAPPENS IF WE
REMOVE TABS AND
SPACES?

29. Diving deeper into linear regression
Full model with the information on tabs and spaces
included
Reduced model without the information on tabs and
spaces

30. Coefficient of determination
how much variance in salary can the model explain
Model Coefficient of determination Adjusted
Full model 0.4008 0.3892
Reduced model 0.3938 0.3892
R2 R2
adj

31. COLLINEARITY?

32. WHAT CHANGED IN THE
REDUCED MODEL?

33. MORE SIGNIFICANT IN
THE REDUCED MODEL
Years of programming experience
Contributing to open source
PHP

34. OPEN SOURCE?

36. Open source contributors use spaces
more than tabs

38. POTENTIAL
EXPLANATION?

39. Language effects?

40. Language effects?

41. LANGUAGE AND OPEN
SOURCE?

42. TABS, SPACES, OPEN
SOURCE & SALARY
How does it fit together?

44. Exploring salary distributions
BASED ON EXPERIENCE LEVEL

45. What’s different for these users?
… more statistical testing

47. The importance of version control
Higher salary Lower salary
Git 168 660
I use some other system 17 30
Subversion 4 47
Team Foundation Server 6 92

48. VERSION CONTROL
AND TABS/SPACES
##
## Pearson's Chi-squared test
##
## data: .
## X-squared = 258.48, df = 18, p-value < 2.2e-16

50. Version control and salary

54. Git and Subversion

55. WHY IS VERSION
CONTROL SO
IMPORTANT?

57. github.com/evelinag/tabs-spaces-talk

58. BUT CAN WE TRUST THE
DATA?

59. Salary distribution

63. WHAT’S WRONG?

67. MISSING DATA

68. Statistics of missing data
missing completely at random
missing at random
missing not at random

69. DATA TRAPS
missing data
wrong data
accidentally or deliberately

72. CAN YOU TRUST DATA?

74. INTERPRETATION
Machine learning as a service

76. EVELINA GABASOVA
Consulting data detective
@evelgab
evelinag.com