Our group has written a lot on calibration of models and even conditional calibration of models.

In our last note we mentioned the possibility of “fully calibrated models.” This note is an example of a probability model that is calibrated in the traditional sense, but not fully calibrated in a finer grained sense.

First let’s attach our packages and generate our example data in `R`

.

```
d <- build_frame(
"x1" , "x2", "y" |
1 , 1 , TRUE |
1 , 0 , FALSE |
1 , 0 , TRUE |
1 , 1 , FALSE |
0 , 0 , TRUE |
0 , 1 , TRUE |
0 , 1 , FALSE |
0 , 0 , FALSE |
0 , 0 , TRUE )
# cat(wrapr::draw_frame(d))
knitr::kable(d)
```

x1 | x2 | y |
---|---|---|

1 | 1 | TRUE |

1 | 0 | FALSE |

1 | 0 | TRUE |

1 | 1 | FALSE |

0 | 0 | TRUE |

0 | 1 | TRUE |

0 | 1 | FALSE |

0 | 0 | FALSE |

0 | 0 | TRUE |

Now, we fit our logistic regression model.

```
##
## Call:
## glm(formula = y ~ x1 + x2, family = binomial(), data = d)
##
## Deviance Residuals:
## Min 1Q Median 3Q Max
## -1.4213 -1.2572 0.9517 1.0996 1.2572
##
## Coefficients:
## Estimate Std. Error z value Pr(>|z|)
## (Intercept) 0.5572 1.0784 0.517 0.605
## x1 -0.3715 1.3644 -0.272 0.785
## x2 -0.3715 1.3644 -0.272 0.785
##
## (Dispersion parameter for binomial family taken to be 1)
##
## Null deviance: 12.365 on 8 degrees of freedom
## Residual deviance: 12.201 on 6 degrees of freedom
## AIC: 18.201
##
## Number of Fisher Scoring iterations: 4
```

We land our model predictions as a new column.

x1 | x2 | y | prediction |
---|---|---|---|

1 | 1 | TRUE | 0.4537010 |

1 | 0 | FALSE | 0.5462990 |

1 | 0 | TRUE | 0.5462990 |

1 | 1 | FALSE | 0.4537010 |

0 | 0 | TRUE | 0.6358007 |

0 | 1 | TRUE | 0.5462990 |

0 | 1 | FALSE | 0.5462990 |

0 | 0 | FALSE | 0.6358007 |

0 | 0 | TRUE | 0.6358007 |

We can see this model is calibrated or unbiased in the sense `E[prediction] = E[outcome]`

.

x | |
---|---|

y | 0.5555556 |

prediction | 0.5555556 |

And it is even calibrated in the sense we expect for logistic regression, `E[prediction * x] = E[outcome * x]`

(where `x`

is any explanatory variable).

```
for(v in qc(x1, x2)) {
print(paste0(
v, ' diff: ',
mean(d[[v]] * d$prediction) - mean(d[[v]] * d$y)))
}
```

```
## [1] "x1 diff: 2.77555756156289e-17"
## [1] "x2 diff: 5.55111512312578e-17"
```

However, we can see this model is not “fully calibrated” in an additional sense requiring that `E[outcome | prediction] = prediction`

for all observed values of `prediction`

.

prediction | y |
---|---|

0.4537010 | 0.5000000 |

0.5462990 | 0.5000000 |

0.6358007 | 0.6666667 |

We can re-calibrate such a model (in practice you would want to do this on out of sample data using isotonic regression, or using cross-frame methods to avoid nested model bias).

```
cal_map <- cal$prediction := cal$y
d$calibrated <- cal_map[as.character(d$prediction)]
knitr::kable(d)
```

x1 | x2 | y | prediction | calibrated |
---|---|---|---|---|

1 | 1 | TRUE | 0.4537010 | 0.5000000 |

1 | 0 | FALSE | 0.5462990 | 0.5000000 |

1 | 0 | TRUE | 0.5462990 | 0.5000000 |

1 | 1 | FALSE | 0.4537010 | 0.5000000 |

0 | 0 | TRUE | 0.6358007 | 0.6666667 |

0 | 1 | TRUE | 0.5462990 | 0.5000000 |

0 | 1 | FALSE | 0.5462990 | 0.5000000 |

0 | 0 | FALSE | 0.6358007 | 0.6666667 |

0 | 0 | TRUE | 0.6358007 | 0.6666667 |

This new calibrated prediction is also calibrated in the standard sense.

x | |
---|---|

y | 0.5555556 |

prediction | 0.5555556 |

calibrated | 0.5555556 |

And, at least in this case, still obeys the explanatory roll-up conditions.

```
for(v in qc(x1, x2)) {
print(paste0(
v, ' diff: ',
mean(d[[v]] * d$calibrated) - mean(d[[v]] * d$y)))
}
```

```
## [1] "x1 diff: 0"
## [1] "x2 diff: 0"
```

The new calibrated predictions are even of lower deviance than the original predictions.

```
deviance <- function(prediction, truth) {
sum(-2 * (truth * log(prediction) +
(1 - truth) * log(1 - prediction)))
}
```

`## [1] 12.20102`

`## [1] 12.13685`

The reason the original logistic model could not make the calibrated predictions is: the calibrated predictions are not a linear function of the explanatory variables in link space.

Categories: data science Statistics

### jmount

Data Scientist and trainer at Win Vector LLC. One of the authors of Practical Data Science with R.

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