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I've noticed that when running a piece multiple times, the gbm produced (see below) produces slightly different results when viewing the summary.

Should that be expected? I.e. running gbm regression is not consistent? 

file.gbm <- gbm(formula, data = my_data
            , distribution = 'bernoulli'
            , shrinkage = 0.01
            , n.minobsinnode = 30
            , interaction.depth = 3
            , n.trees = 500)
summary(file.gbm)
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1 Answer 1

Reset to default
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In GBM you have an option called bag.fraction. The help file specifies this as following:

the fraction of the training set observations randomly selected to propose the next tree in the expansion. This introduces randomnesses into the model fit. If bag.fraction<1 then running the same model twice will result in similar but different fits. gbm uses the R random number generator so set.seed can ensure that the model can be reconstructed.

The default setting in GBM is bag.fraction = 0.5. If you want your gbm call to return the same result, use set.seed before calling gbm.

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  • $\begingroup$ In other words, this is stochastic gradient boosting if bag.fraction < 1? $\endgroup$
    – Mysterious Otter
    Commented Oct 28, 2016 at 8:27

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