# histogram_type¶

• Available in: GBM, DRF
• Hyperparameter: yes

## Description¶

Histogram aggregation is commonly used to speed up the split finding process in decision tree algorithms. Instead of considering every possible way to split a set of training instances based on a given feature, the problem can be simplified by considering discrete sections of the feature space.

For example if we have some feature ranging from 0-100 and 10M training instances, a traditional tree algorithm will enumerate over all 10M training instances to find a split. Instead the feature space could be evenly divided into 10 regions (0-10,10-20,…,90-100). The information from training instances is then aggregated into these histogram bins, and the process of finding a split now only requires enumerating over 10 possible split points.

The histogram_type option specifies to the method of calculating these bin boundaries. The option has an impact on the possible split points a tree algorithm is able to select.

By default (AUTO) GBM/DRF bins from min…max in steps of (max-min)/N. Use this option to specify the type of histogram to use for finding optimal split points. Available types include:

• AUTO
• Random
• QuantilesGlobal
• RoundRobin

When histogram_type="UniformAdaptive" is specified, each feature is binned into buckets of equal step size (not population). This is the fastest method, and usually performs well, but can lead to less accurate splits if the distribution is highly skewed.

H2O supports extremely randomized trees (XRT) via histogram_type="Random". When this is specified, the algorithm will sample N-1 points from min…max and use the sorted list of those to find the best split. The cut points are random rather than uniform. For example, to generate 4 bins for some feature ranging from 0-100, 3 random numbers would be generated in this range (13.2, 89.12, 45.0). The sorted list of these random numbers forms the histogram bin boundaries e.g. (0-13.2, 13.2-45.0, 45.0-89.12, 89.12-100).

When histogram_type="QuantilesGlobal" is specified, the feature distribution is taken into account with a quantile-based binning (where buckets have equal population). This computes nbins quantiles for each numeric (non-binary) column, then refines/pads each bucket (between two quantiles) uniformly (and randomly for remainders) into a total of nbins_top_level bins. This set of split points is then used for all levels of the tree: each leaf node histogram gets min/max-range adjusted (based on its population range) and also linearly refined/padded to end up with exactly nbins (level) bins to pick the best split from. For integer columns where this ends up with more than the unique number of distinct values, the algorithm falls back to the pure-integer buckets.

When histogram_type="RoundRobin" is specified, the algorithm will cycle through all histogram types (one per tree).

## Example¶

library(h2o)
h2o.init()
# import the airlines dataset:
# This dataset is used to classify whether a flight will be delayed 'YES' or not "NO"
# original data can be found at http://www.transtats.bts.gov/

# convert columns to factors
airlines["Year"] <- as.factor(airlines["Year"])
airlines["Month"] <- as.factor(airlines["Month"])
airlines["DayOfWeek"] <- as.factor(airlines["DayOfWeek"])
airlines["Cancelled"] <- as.factor(airlines["Cancelled"])
airlines['FlightNum'] <- as.factor(airlines['FlightNum'])

# set the predictor names and the response column name
predictors <- c("Origin", "Dest", "Year", "UniqueCarrier", "DayOfWeek", "Month", "Distance", "FlightNum")
response <- "IsDepDelayed"

# split into train and validation
airlines.splits <- h2o.splitFrame(data =  airlines, ratios = .8, seed = 1234)
train <- airlines.splits[[1]]
valid <- airlines.splits[[2]]

# try using the histogram_type parameter:
airlines.gbm <- h2o.gbm(x = predictors, y = response, training_frame = train,
validation_frame = valid, histogram_type = "UniformAdaptive" ,
seed = 1234)

# print the AUC for the validation data
print(h2o.auc(airlines.gbm, valid = TRUE))

# Example of values to grid over for histogram_type
hyper_params <- list( histogram_type = c("UniformAdaptive", "Random", "QuantilesGlobal", "RoundRobin") )

# this example uses cartesian grid search because the search space is small
# and we want to see the performance of all models. For a larger search space use
# random grid search instead: list(strategy = "RandomDiscrete")
# this GBM uses early stopping once the validation AUC doesn't improve by at least 0.01% for
# 5 consecutive scoring events
grid <- h2o.grid(x = predictors, y = response, training_frame = train, validation_frame = valid,
algorithm = "gbm", grid_id = "air_grid", hyper_params = hyper_params,
stopping_rounds = 5, stopping_tolerance = 1e-4, stopping_metric = "AUC",
search_criteria = list(strategy = "Cartesian"), seed = 1234)

## Sort the grid models by AUC
sortedGrid <- h2o.getGrid("air_grid", sort_by = "auc", decreasing = TRUE)
sortedGrid

import h2o
h2o.init()

# import the airlines dataset:
# This dataset is used to classify whether a flight will be delayed 'YES' or not "NO"
# original data can be found at http://www.transtats.bts.gov/

# convert columns to factors
airlines["Year"]= airlines["Year"].asfactor()
airlines["Month"]= airlines["Month"].asfactor()
airlines["DayOfWeek"] = airlines["DayOfWeek"].asfactor()
airlines["Cancelled"] = airlines["Cancelled"].asfactor()
airlines['FlightNum'] = airlines['FlightNum'].asfactor()

# set the predictor names and the response column name
predictors = ["Origin", "Dest", "Year", "UniqueCarrier", "DayOfWeek", "Month", "Distance", "FlightNum"]
response = "IsDepDelayed"

# split into train and validation sets
train, valid= airlines.split_frame(ratios = [.8], seed = 1234)

# try using the histogram_type parameter:

airlines_gbm.train(x = predictors, y = response, training_frame = train, validation_frame = valid)

# print the auc for the validation data
print(airlines_gbm.auc(valid=True))

# Example of values to grid over for histogram_type
# import Grid Search
from h2o.grid.grid_search import H2OGridSearch

# select the values for histogram_type to grid over
hyper_params = {'histogram_type': ["UniformAdaptive", "Random", "QuantilesGlobal", "RoundRobin"]}

# this example uses cartesian grid search because the search space is small
# and we want to see the performance of all models. For a larger search space use
# random grid search instead: {'strategy': "RandomDiscrete"}
# initialize the GBM estimator
# use early stopping once the validation AUC doesn't improve by at least 0.01% for
# 5 consecutive scoring events
stopping_rounds = 5,
stopping_metric = "AUC", stopping_tolerance = 1e-4)

# build grid search with previously made GBM and hyper parameters
grid = H2OGridSearch(model = airlines_gbm_2, hyper_params = hyper_params,
search_criteria = {'strategy': "Cartesian"})

# train using the grid
grid.train(x = predictors, y = response, training_frame = train, validation_frame = valid)

# sort the grid models by decreasing AUC
sorted_grid = grid.get_grid(sort_by = 'auc', decreasing = True)
print(sorted_grid)