# Local Global Trend (LGT)¶

In this section, we will cover:

• LGT model structure

• difference between DLT and LGT

• syntax to call LGT classes with different estimation methods

LGT stands for Local and Global Trend and is a refined model from Rlgt (Smyl et al., 2019). The main difference is that LGT is an additive form taking log-transformation response as the modeling response. This essentially converts the model into a multicplicative with some advantages (Ng and Wang et al., 2020). However, one drawback of this approach is that negative response values are not allowed due to the existence of the global trend term and because of that we start to deprecate the support of regression of this model.

:

%matplotlib inline

import pandas as pd
import numpy as np
import matplotlib.pyplot as plt

import orbit
from orbit.models.lgt import LGTMAP, LGTAggregated, LGTFull
from orbit.diagnostics.plot import plot_predicted_data
from orbit.diagnostics.plot import plot_predicted_components
from orbit.utils.plot import get_orbit_style
plt.style.use(get_orbit_style())

:

print(orbit.__version__)

1.0.17


## Model Structure¶

\begin{split}\begin{align*} y_{t} &= \mu_t + s_t + \epsilon_t \\ \mu_t &= l_{t-1} + \xi_1 b_{t-1} + \xi_2 l_{t-1}^{\lambda}\\ \epsilon_t &~\sim \mathtt{Student}(\nu, 0, \sigma)\\ \sigma &~\sim \mathtt{HalfCauchy}(0, \gamma_0) \end{align*}\end{split}

with the update process,

\begin{split}\begin{align*} l_t &= \rho_l(y_t - s_t) + (1-\rho_l)l_{t-1}\\ b_t &= \rho_b(l_t - l_{t-1}) + (1-\rho_b)b_{t-1}\\ s_{t+m} &= \rho_s(y_t - l_t ) + (1-\rho_s)s_t \end{align*}\end{split}

Unlike DLT model which has a deterministic trend, LGT introduces a hybrid trend where it consists of

• local trend takes on a fraction $$\xi_1$$ rather than a damped factor

• global trend is with a auto-regrssive term $$\xi_2$$ and a power term $$\lambda$$

We will continue to use the iclaims data with 52 weeks train-test split.

:

# load data
# define date and response column
date_col = 'week'
response_col = 'claims'
df.dtypes
test_size = 52
train_df = df[:-test_size]
test_df = df[-test_size:]


## LGT Model¶

In orbit, we have three types of LGT models, LGTMAP, LGTAggregated and LGTFull. Orbit follows a sklearn style model API. We can create an instance of the Orbit class and then call its fit and predict methods.

### LGTMAP¶

LGTMAP is the model class for MAP (Maximum a Posteriori) estimation.

:

lgt = LGTMAP(
response_col=response_col,
date_col=date_col,
seasonality=52,
seed=8888,
)

:

%%time
lgt.fit(df=train_df)

CPU times: user 202 ms, sys: 9.81 ms, total: 211 ms
Wall time: 504 ms

:

predicted_df = lgt.predict(df=test_df)

:

_ = plot_predicted_data(training_actual_df=train_df, predicted_df=predicted_df,
date_col=date_col, actual_col=response_col,
test_actual_df=test_df, title='Prediction with LGTMAP Model') ### LGTFull¶

LGTFull is the model class for full Bayesian prediction. In full Bayesian prediction, the prediction will be conducted once for each parameter posterior sample, and the final prediction results are aggregated. Prediction will always return the median (aka 50th percentile) along with any additional percentiles that are provided.

:

lgt = LGTFull(
response_col=response_col,
date_col=date_col,
seasonality=52,
seed=8888,
)

:

%%time
lgt.fit(df=train_df)

WARNING:pystan:Maximum (flat) parameter count (1000) exceeded: skipping diagnostic tests for n_eff and Rhat.
To run all diagnostics call pystan.check_hmc_diagnostics(fit)
WARNING:pystan:1 of 100 iterations ended with a divergence (1 %).
WARNING:pystan:Try running with adapt_delta larger than 0.8 to remove the divergences.

CPU times: user 61.2 ms, sys: 61.6 ms, total: 123 ms
Wall time: 6.32 s

:

predicted_df = lgt.predict(df=test_df)

:

predicted_df.tail(5)

:

week prediction_5 prediction prediction_95
47 2018-05-27 12.099949 12.232984 12.330652
48 2018-06-03 12.060341 12.173674 12.293869
49 2018-06-10 12.118473 12.262561 12.408782
50 2018-06-17 12.097858 12.239122 12.341881
51 2018-06-24 12.193468 12.281816 12.383324
:

_ = plot_predicted_data(training_actual_df=train_df, predicted_df=predicted_df,
date_col=lgt.date_col, actual_col=lgt.response_col,
test_actual_df=test_df, title='Prediction with LGTFull Model') ### LGTAggregated¶

LGTAggregated is the model class for aggregated posterior prediction. In aggregated prediction, the parameter posterior samples are reduced using aggregate_method ({ 'mean', 'median' }) before performing a single prediction.

:

lgt = LGTAggregated(
response_col=response_col,
date_col=date_col,
seasonality=52,
seed=8888,
)

:

%%time
lgt.fit(df=train_df)

WARNING:pystan:Maximum (flat) parameter count (1000) exceeded: skipping diagnostic tests for n_eff and Rhat.
To run all diagnostics call pystan.check_hmc_diagnostics(fit)
WARNING:pystan:1 of 100 iterations ended with a divergence (1 %).
WARNING:pystan:Try running with adapt_delta larger than 0.8 to remove the divergences.

CPU times: user 69 ms, sys: 67.8 ms, total: 137 ms
Wall time: 6.55 s

:

predicted_df = lgt.predict(df=test_df)

:

predicted_df.tail(5)

:

week prediction_5 prediction prediction_95
47 2018-05-27 12.091428 12.204437 12.313660
48 2018-06-03 12.031525 12.139519 12.250433
49 2018-06-10 12.124013 12.233509 12.345642
50 2018-06-17 12.090025 12.200898 12.311468
51 2018-06-24 12.138503 12.247008 12.357515
:

_ = plot_predicted_data(training_actual_df=train_df, predicted_df=predicted_df,
date_col=lgt.date_col, actual_col=lgt.response_col,
test_actual_df=test_df, title='Predictibon with LGTAggregated Model') More details for each method are available in the docstrings and also here: https://uber.github.io/orbit/orbit.models.html#module-orbit.models.lgt