testZeroInflation | R Documentation |
This function compares the observed number of zeros with the zeros expected from simulations.
testZeroInflation(simulationOutput, ...)
simulationOutput |
an object of class DHARMa, either created via |
... |
further arguments to |
The plot shows the expected distribution of zeros against the observed values, the ratioObsSim shows observed vs. simulated zeros. A value < 1 means that the observed data has less zeros than expected, a value > 1 means that it has more zeros than expected (aka zero-inflation). Per default, the function tests both sides.
Some notes about common problems / questions:
Zero-inflation tests after fitting the model are crucial to see if you have zero-inflation. Just because there are a lot of zeros doesn't mean you have zero-inflation, see Warton, D. I. (2005). Many zeros does not mean zero inflation: comparing the goodness-of-fit of parametric models to multivariate abundance data. Environmetrics 16(3), 275-289.
That being said, zero-inflation tests are often not a reliable guide to decide wheter to add a zi term or not. In general, model structures should be decided on ideally a priori, if that is not possible via model selection techniques (AIC, BIC, WAIC, Bayes Factor). A zero-inflation test should only be run after that decision, and to validate the decision that was taken.
This function is a wrapper for testGeneric
, where the summary argument is set to function(x) sum(x == 0)
Florian Hartig
testResiduals
, testUniformity
, testOutliers
, testDispersion
, testZeroInflation
, testGeneric
, testTemporalAutocorrelation
, testSpatialAutocorrelation
, testQuantiles
, testCategorical
testData = createData(sampleSize = 100, overdispersion = 0.5, randomEffectVariance = 0) fittedModel <- glm(observedResponse ~ Environment1 , family = "poisson", data = testData) simulationOutput <- simulateResiduals(fittedModel = fittedModel) # the plot function runs 4 tests # i) KS test i) Dispersion test iii) Outlier test iv) quantile test plot(simulationOutput, quantreg = TRUE) # testResiduals tests distribution, dispersion and outliers # testResiduals(simulationOutput) ####### Individual tests ####### # KS test for correct distribution of residuals testUniformity(simulationOutput) # KS test for correct distribution within and between groups testCategorical(simulationOutput, testData$group) # Dispersion test - for details see ?testDispersion testDispersion(simulationOutput) # tests under and overdispersion # Outlier test (number of observations outside simulation envelope) # Use type = "boostrap" for exact values, see ?testOutliers testOutliers(simulationOutput, type = "binomial") # testing zero inflation testZeroInflation(simulationOutput) # testing generic summaries countOnes <- function(x) sum(x == 1) # testing for number of 1s testGeneric(simulationOutput, summary = countOnes) # 1-inflation testGeneric(simulationOutput, summary = countOnes, alternative = "less") # 1-deficit means <- function(x) mean(x) # testing if mean prediction fits testGeneric(simulationOutput, summary = means) spread <- function(x) sd(x) # testing if mean sd fits testGeneric(simulationOutput, summary = spread)
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