Advanced Topics¶

Missing Value Handle¶

LightGBM enables the missing value handle by default. Disable it by setting use_missing=false.
LightGBM uses NA (NaN) to represent missing values by default. Change it to use zero by setting zero_as_missing=true.
When zero_as_missing=false (default), the unshown values in sparse matrices (and LightSVM) are treated as zeros.
When zero_as_missing=true, NA and zeros (including unshown values in sparse matrices (and LightSVM)) are treated as missing.

LightGBM offers good accuracy with integer-encoded categorical features. LightGBM applies Fisher (1958) to find the optimal split over categories as described here. This often performs better than one-hot encoding.
Use categorical_feature to specify the categorical features. Refer to the parameter categorical_feature in Parameters.
Categorical features must be encoded as non-negative integers (int) less than Int32.MaxValue (2147483647). It is best to use a contiguous range of integers started from zero.
Use min_data_per_group, cat_smooth to deal with over-fitting (when #data is small or #category is large).
For a categorical feature with high cardinality (#category is large), it often works best to treat the feature as numeric, either by simply ignoring the categorical interpretation of the integers or by embedding the categories in a low-dimensional numeric space.

The label should be of type int, such that larger numbers correspond to higher relevance (e.g. 0:bad, 1:fair, 2:good, 3:perfect).
Use label_gain to set the gain(weight) of int label.
Use max_position to set the NDCG optimization position.