Hi to all,

I have problems fitting the following gamma model:

```
# Library imports
library(data.table)
library(simstudy)
library(brms)
#> Lade nötiges Paket: Rcpp
#> Loading 'brms' package (version 2.9.0). Useful instructions
#> can be found by typing help('brms'). A more detailed introduction
#> to the package is available through vignette('brms_overview').
# Define the data
def <- defData(varname = "male", formula = .5, dist = "binary", id = "eye_id")
def <- defData(def, varname = "oct", formula = .5, dist = "binary")
defC <- defDataAdd(varname = "thickness",
formula = "390 + 8 * male - 10 * oct",
variance = 66^2
)
defC <- defDataAdd(defC, varname = "pressure",
formula = 2.86,
variance = .563^2,
dist = "gamma",
link = "identity")
# Generate the data
set.seed(41948)
dd <- genData(1000, def)
dd <- addCorFlex(dd, defC, rho = .6, corstr = "cs")
dd
#> eye_id male oct thickness pressure
#> 1: 1 1 0 461.6095 2.3230065
#> 2: 2 0 1 382.8064 2.9249466
#> 3: 3 1 1 450.2778 1.9641713
#> 4: 4 0 0 368.7375 1.2235662
#> 5: 5 0 0 431.0898 1.8071625
#> ---
#> 996: 996 1 1 314.1276 1.3455995
#> 997: 997 0 1 413.1119 2.3405988
#> 998: 998 0 0 203.4344 0.7652045
#> 999: 999 1 1 280.7003 0.9592640
#> 1000: 1000 0 0 419.2663 4.4517026
# Check means, sd, and correlation
dd[, .(mean(male), mean(oct))]
#> V1 V2
#> 1: 0.493 0.502
dd[, .(mean(thickness), sd(thickness)), keyby = .(male, oct)]
#> male oct V1 V2
#> 1: 0 0 389.3978 68.68572
#> 2: 0 1 382.5514 69.09854
#> 3: 1 0 403.4099 66.39633
#> 4: 1 1 388.8900 67.21697
dd[, .(mean(pressure), sd(pressure)), keyby = .(male, oct)]
#> male oct V1 V2
#> 1: 0 0 2.985553 1.689790
#> 2: 0 1 2.813059 1.590758
#> 3: 1 0 2.990881 1.659774
#> 4: 1 1 2.954744 1.616318
dd[, .(mean(pressure), sd(pressure))]
#> V1 V2
#> 1: 2.936086 1.638918
dd[, cor(thickness, pressure)]
#> [1] 0.5869474
# Fit Gamma model
fit <- brm(pressure ~ thickness + male + oct, family = Gamma(link = "identity"), data = dd)
#> Compiling the C++ model
#> Start sampling
#>
#> SAMPLING FOR MODEL '0b1a6aa24224242a80a515bd48d206ca' NOW (CHAIN 1).
#> Chain 1: Rejecting initial value:
#> Chain 1: Error evaluating the log probability at the initial value.
#> Chain 1: Exception: gamma_lpdf: Inverse scale parameter[1] is -0.071384, but must be > 0! (in 'modeldeea13a01997_0b1a6aa24224242a80a515bd48d206ca' at line 37)
#>
#> Chain 1: Rejecting initial value:
#> Chain 1: Error evaluating the log probability at the initial value.
#> Chain 1: Exception: gamma_lpdf: Inverse scale parameter[2] is -0.164441, but must be > 0! (in 'modeldeea13a01997_0b1a6aa24224242a80a515bd48d206ca' at line 37)
#>
#> Chain 1: Rejecting initial value:
#> Chain 1: Error evaluating the log probability at the initial value.
#> Chain 1: Exception: gamma_lpdf: Inverse scale parameter[2] is -0.0536331, but must be > 0! (in 'modeldeea13a01997_0b1a6aa24224242a80a515bd48d206ca' at line 37)
#>
#> Chain 1: Rejecting initial value:
#> Chain 1: Error evaluating the log probability at the initial value.
#> Chain 1: Exception: gamma_lpdf: Inverse scale parameter[1] is -0.0102431, but must be > 0! (in 'modeldeea13a01997_0b1a6aa24224242a80a515bd48d206ca' at line 37)
#>
#> Chain 1: Rejecting initial value:
#> Chain 1: Error evaluating the log probability at the initial value.
#> Chain 1: Exception: gamma_lpdf: Inverse scale parameter[2] is -0.981855, but must be > 0! (in 'modeldeea13a01997_0b1a6aa24224242a80a515bd48d206ca' at line 37)
#>
#> Chain 1: Rejecting initial value:
#> Chain 1: Error evaluating the log probability at the initial value.
#> Chain 1: Exception: gamma_lpdf: Inverse scale parameter[1] is -0.0306889, but must be > 0! (in 'modeldeea13a01997_0b1a6aa24224242a80a515bd48d206ca' at line 37)
.
.
.
#>
#> Chain 1:
#> Chain 1: Initialization between (-2, 2) failed after 100 attempts.
#> Chain 1: Try specifying initial values, reducing ranges of constrained values, or reparameterizing the model.
#> Error in sampler$call_sampler(args_list[[i]]) : Initialization failed.
#> character(0)
#> error occurred during calling the sampler; sampling not done
```

^{Created on 2019-09-02 by the reprex package (v0.2.1)}

I tried setting the initial values to “0”, but this does not help understandably (according to the error message).

How can I start the sampler?

Thanks and kind regards,

Pascal

Please also provide the following information in addition to your question:

- Operating System: macOS Sierra 10.12.6
- brms Version: 2.9.0