Goodness of fit tests for distance sampling models
Generic function that computes chi-square goodness of fit test for detection function models with binned data and Cramer-von Mises and Kolmogorov-Smirnov (if ks=TRUE)tests for exact distance data. By default a Q-Q plot is generated for exact data (and can be suppressed using the qq=FALSE argument).
ddf.gof( model, breaks = NULL, nc = NULL, qq = TRUE, nboot = 100, ks = FALSE, ... )
model |
model object |
breaks |
Cutpoints to use for binning data |
nc |
Number of distance classes |
qq |
Flag to indicate whether quantile-quantile plot is desired |
nboot |
number of replicates to use to calculate p-values for the Kolmogorov-Smirnov goodness of fit test statistics |
ks |
perform the Kolmogorov-Smirnov test (this involves many bootstraps so can take a while) |
... |
Graphics parameters to pass into qqplot function |
Formal goodness of fit testing for detection function models using Kolmogorov-Smirnov and Cramer-von Mises tests. Both tests are based on looking at the quantile-quantile plot produced by qqplot.ddf and deviations from the line x=y.
The Kolmogorov-Smirnov test asks the question "what's the largest vertical distance between a point and the y=x line?" It uses this distance as a statistic to test the null hypothesis that the samples (EDF and CDF in our case) are from the same distribution (and hence our model fits well). If the deviation between the y=x line and the points is too large we reject the null hypothesis and say the model doesn't have a good fit.
Rather than looking at the single biggest difference between the y=x line and the points in the Q-Q plot, we might prefer to think about all the differences between line and points, since there may be many smaller differences that we want to take into account rather than looking for one large deviation. Its null hypothesis is the same, but the statistic it uses is the sum of the deviations from each of the point to the line.
List of class ddf.gof containing
chi-square |
Goodness of fit test statistic |
df |
Degrees of freedom associated with test statistic |
p-value |
Significance level of test statistic |
Note that a bootstrap procedure is required for the Kolmogorov-Smirnov test to ensure that the p-values from the procedure are correct as the we are comparing the cumulative distribution function (CDF) and empirical distribution function (EDF) and we have estimated the parameters of the detection function. The nboot parameter controls the number of bootstraps to use. Set to 0 to avoid computing bootstraps (much faster but with no Kolmogorov-Smirnov results, of course).
Jeff Laake
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