# π Wrap-up quizΒΆ

**This quiz requires some programming to be answered.**

Open the dataset `bike_rides.csv`

with the following commands:

```
import pandas as pd
cycling = pd.read_csv("../datasets/bike_rides.csv", index_col=0,
parse_dates=True)
cycling.index.name = ""
target_name = "power"
data, target = cycling.drop(columns=target_name), cycling[target_name]
data.head()
```

A detailed description of this dataset is given in the appendix. As a reminder, the problem we are trying to solve with this dataset is to use measurements from cheap sensors (GPS, heart-rate monitor, etc.) in order to predict a cyclist power. Power can indeed be recorded via a cycling power meter device, but this device is rather expensive.

Instead of using blindly machine learning, we will first introduce some flavor of classic mechanics: the Newtonβs second law.

\(P_{meca} = (\frac{1}{2} \rho . SC_x . V_{a}^{2} + C_r . mg . \cos \alpha + mg . \sin \alpha + ma) V_d\)

where \(\rho\) is the air density in kg.m\(^{-3}\), \(S\) is frontal surface of the cyclist in m\(^{2}\), \(C_x\) is the drag coefficient, \(V_a\) is the air speed in m.s\(^{-1}\), \(C_r\) is the rolling coefficient, \(m\) is the mass of the rider and bicycle in kg, \(g\) is the standard acceleration due to gravity which is equal to 9.81 m.s\(^{-2}\), \(\alpha\) is the slope in radian, \(V_d\) is the rider speed in m.s\(^{-1}\), and \(a\) is the rider acceleration in m.s\(^{-2}\).

This equation might look a bit complex at first but we can explain with words what the different terms within the parenthesis are:

the first term is the power that a cyclist is required to produce to fight wind

the second term is the power that a cyclist is required to produce to fight the rolling resistance created by the tires on the floor

the third term is the power that a cyclist is required to produce to go up a hill if the slope is positive. If the slope is negative the cyclist does not need to produce any power to go forward

the fourth and last term is the power that a cyclist requires to change his speed (i.e. acceleration).

We can simplify the model above by using the data that we have at hand. It would look like the following.

\(P_{meca} = \beta_{1} V_{d}^{3} + \beta_{2} V_{d} + \beta_{3} \sin(\alpha) V_{d} + \beta_{4} a V_{d}\)

This model is closer to what we saw previously: it is a linear model trained on a non-linear feature transformation. We will build, train and evaluate such a model as part of this exercise. Thus, you need to:

create a new data matrix containing the cube of the speed, the speed, the speed multiplied by the sine of the angle of the slope, and the speed multiplied by the acceleration. To compute the angle of the slope, you need to take the arc tangent of the slope (

`alpha = np.arctan(slope)`

). In addition, we can limit ourself to positive acceleration only by clipping to 0 the negative acceleration values (they would correspond to some power created by the braking that we are not modeling here).using the new data matrix, create a linear predictive model based on a

`sklearn.preprocessing.StandardScaler`

and a`sklearn.linear_model.RidgeCV`

;use a

`sklearn.model_selection.ShuffleSplit`

cross-validation strategy with only 4 splits (`n_splits=4`

) to evaluate the statistical performance of the model. Use the mean absolute error (MAE) as a statistical performance metric. Also, pass the parameter`return_estimator=True`

and`return_train_score=True`

to answer the subsequent questions. Be aware that the`ShuffleSplit`

strategy is a naive strategy and we will investigate the consequence of making this choice in the subsequent questions.

Question

What is the mean value of the column containing the information of \(\sin(\alpha) V_{d}\)?

a) about -3

b) about -0.3

c) about -0.03

d) about -0.003

Question

On average, the Mean Absolute Error on the test sets obtained through cross-validation is closest to:

a) 20 Watts

b) 50 Watts

c) 70 Watts

d) 90 Watts

*Select a single answer*

Hint: pass `scoring="neg_mean_absolute_error"`

to the `cross_validate`

function to compute the (negative of) the requested metric.
Hint: it is possible to replace the negative acceleration values by 0 using
`data["acceleration"].clip(lower=0)`

Question

Given the model \(P_{meca} = \beta_{1} V_{d}^{3} + \beta_{2} V_{d} + \beta_{3} \sin(\alpha) V_{d} + \beta_{4} a V_{d}\) that you created, inspect the weights of the linear models fitted during cross-validation and select the correct statements:

a) \(\beta_{1} < \beta_{2} < \beta_{3}\)

b) \(\beta_{3} < \beta_{1} < \beta_{2}\)

c) \(\beta_{2} < \beta_{3} < \beta_{1}\)

d) \(\beta_{1} < 0\)

e) \(\beta_{2} < 0\)

f) \(\beta_{3} < 0\)

g) \(\beta_{4} < 0\)

h) All \(\beta\)s are \(> 0\)

*Select several answers*

Now, we will create a predictive model that uses all available sensor
measurements such as cadence (the speed at which a cyclist turns pedals
measured in rotation per minute) and heart-rate (the number of beat per minute
of the heart of the cyclist while exercising). Also, we will use a non-linear
regressor, a
`sklearn.ensemble.HistGradientBoostingRegressor`

.
Fix the number of maximum iterations to 1000 (`max_iter=1_000`

) and activate
the early stopping (`early_stopping=True`

). Repeat the previous evaluation
using this regressor.

Question

On average, the Mean Absolute Error on the test sets obtained through cross-validation is closest to:

a) 20 Watts

b) 40 Watts

c) 60 Watts

d) 80 Watts

*Select a single answer*

Question

Comparing both the linear model and the histogram gradient boosting model and taking into consideration the train and test MAE obtained via cross-validation, select the correct statements:

a) the statistical performance of the histogram gradient-boosting model is limited by its underfitting

b) the statistical performance of the histogram gradient-boosting model is limited by its overfitting

c) the statistical performance of the linear model is limited by its underfitting

d) the statistical performance of the linear model is limited by its overfitting

*Select several answers*

Hint: look at the values of the `train_score`

and the `test_score`

collected
in the dictionaries returned by the `cross_validate`

function.

In the previous cross-validation, we made the choice of using a `ShuffleSplit`

cross-validation strategy. It means that randomly selected samples were
selected as a testing test ignoring any time dependency between the lines of
the dataframe.

We would like to have a cross-validation strategy that evaluates the capacity of our model to predict on a completely new bike ride: the samples in the validation set should only come from rides not present in the training set.

Question

How many bike rides are stored in the dataframe `data`

? Do not hesitate to
look at the hints.

a) 2

b) 3

c) 4

d) 5

*Select a single answer*

Hint: You can check the unique day in the `DatetimeIndex`

(the index of the
dataframe `data`

). Indeed, we assume that on a given day the rider went cycling
at most once per day.
Hint: You can access to the date and time of a `DatetimeIndex`

using
`df.index.date`

and `df.index.time`

, respectively.

Instead of using the naive `ShuffleSplit`

strategy, we will use a strategy that
takes into account the group defined by each individual date. It corresponds to
a bike ride. We would like to have a cross-validation strategy that evaluates
the capacity of our model to predict on a completely new bike ride: the samples
in the validation set should only come from rides not present in the training
set. Therefore, we can use a `LeaveOneGroupOut`

strategy: at each iteration of
the cross-validation, we will keep a bike ride for the evaluation and use all
other bike rides to train our model.

Thus, you concretely need to:

create a variable called

`group`

that is a 1D numpy array containing the index of each ride present in the dataframe. Therefore, the length of`group`

will be equal to the number of samples in`data`

. If we had 2 bike rides, we would expect the indices 0 and 1 in`group`

to differentiate the bike ride. You can use`pd.factorize`

to encode any Python types into integer indices.create a cross-validation object named

`cv`

using the`sklearn.model_selection.LeaveOneGroupOut`

strategy.evaluate both the linear and histogram gradient boosting models with this strategy.

Question

Using the previous evaluations (with both `ShuffleSplit`

and
`LeaveOneGroupOut`

) and looking at the train and test errors for both models,
select the correct statements:

a) the statistical performance of the gradient-boosting model is limited by its underfitting

b) the statistical performance of the gradient-boosting model is limited by its overfitting

c) the statistical performance of the linear model is limited by its underfitting

d) the statistical performance of the linear model is limited by its overfitting

*Select several answers*

Question

Using the previous evaluations (with both `ShuffleSplit`

and
`LeaveOneGroupOut`

) and looking at the train and test errors for both models,
select the correct statements:

a)

`ShuffleSplit`

is giving over-optimistic results for the linear modelb)

`LeaveOneGroupOut`

is giving over-optimistic results for the linear modelc) both cross-validation strategies are equivalent for the linear model

d)

`ShuffleSplit`

is giving over-optimistic results for the gradient-boosting modele)

`LeaveOneGroupOut`

is giving over-optimistic results for the gradient-boosting modelf) both cross-validation strategies are equivalent for the gradient-boosting model

*Select several answer*

Question

Compare more precisely the errors estimated through cross-validation and select the correct statement:

a) in general, the standard deviation of the train and test errors increased using the

`LeaveOneGroupOut`

cross-validationb) in general, the standard deviation of the train and test errors decreased using the

`LeaveOneGroupOut`

cross-validation

*Select a single answer*

Now, we will go in details by picking a single ride for the testing and analyse
the predictions of the models for this test ride. To do so, we can reuse the
`LeaveOneGroupOut`

cross-validation object in the following manner:

```
cv = LeaveOneGroupOut()
train_indices, test_indices = list(cv.split(data, target, groups=groups))[0]
data_linear_model_train = data_linear_model.iloc[train_indices]
data_linear_model_test = data_linear_model.iloc[test_indices]
data_train = data.iloc[train_indices]
data_test = data.iloc[test_indices]
target_train = target.iloc[train_indices]
target_test = target.iloc[test_indices]
```

Now, fit both the linear model and the histogram gradient boosting regressor models on the training data and collect the predictions on the testing data. Make a scatter plot where on the x-axis, you will plot the measured powers (true target) and on the y-axis, you will plot the predicted powers (predicted target). Do two separated plots for each model.

Question

By analysing the plots, select the correct statements:

a) the linear regressor tends to under-predict samples with high power

b) the linear regressor tends to over-predict samples with high power

c) the linear regressor makes catastrophic predictions for samples with low power

d) the histogram gradient boosting regressor tends to under-predict samples with high power

e) the histogram gradient boosting regressor tends to over-predict samples with high power

f) the histogram gradient boosting makes catastrophic predictions for samples with low power

*Select several answers*

Now select a portion of the testing data using the following code:

```
time_slice = slice("2020-08-18 17:00:00", "2020-08-18 17:05:00")
data_test_linear_model_subset = data_linear_model_test[time_slice]
data_test_subset = data_test[time_slice]
target_test_subset = target_test[time_slice]
```

It allows to select data from 5.00 pm until 5.05 pm. Used the previous fitted models (linear and gradient-boosting regressor) to predict on this portion of the test data. Draw on the same plot the true targets and the predictions of each model.

Question

By using the previous plot, select the correct statements:

a) the linear model is more accurate than the histogram gradient boosting regressor

b) the histogram gradient boosting regressor is more accurate than the linear model

c) the linear model predicts smoother outputs than the histogram gradient boosting regressor

d) the histogram gradient boosting regressor predicts smoother outputs than the linear model

*Select several answers*