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Specify a monotonic predictor term in brms. The function does not evaluate its arguments – it exists purely to help set up a model.

Usage

mo(x, id = NA)

Arguments

x

An integer variable or an ordered factor to be modeled as monotonic.

id

Optional character string. All monotonic terms with the same id within one formula will be modeled as having the same simplex (shape) parameter vector. If all monotonic terms of the same predictor have the same id, the resulting predictions will be conditionally monotonic for all values of interacting covariates (Bürkner & Charpentier, 2020).

Details

See Bürkner and Charpentier (2020) for the underlying theory. For detailed documentation of the formula syntax used for monotonic terms, see help(brmsformula) as well as vignette("brms_monotonic").

References

Bürkner P. C. & Charpentier E. (2020). Modeling Monotonic Effects of Ordinal Predictors in Regression Models. British Journal of Mathematical and Statistical Psychology. doi:10.1111/bmsp.12195

See also

Examples

# \dontrun{
# generate some data
income_options <- c("below_20", "20_to_40", "40_to_100", "greater_100")
income <- factor(sample(income_options, 100, TRUE),
                 levels = income_options, ordered = TRUE)
mean_ls <- c(30, 60, 70, 75)
ls <- mean_ls[income] + rnorm(100, sd = 7)
dat <- data.frame(income, ls)

# fit a simple monotonic model
fit1 <- brm(ls ~ mo(income), data = dat)
#> Compiling Stan program...
#> Start sampling
#> 
#> SAMPLING FOR MODEL 'anon_model' NOW (CHAIN 1).
#> Chain 1: 
#> Chain 1: Gradient evaluation took 3.1e-05 seconds
#> Chain 1: 1000 transitions using 10 leapfrog steps per transition would take 0.31 seconds.
#> Chain 1: Adjust your expectations accordingly!
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#> Chain 1:                0.308 seconds (Total)
#> Chain 1: 
#> 
#> SAMPLING FOR MODEL 'anon_model' NOW (CHAIN 2).
#> Chain 2: 
#> Chain 2: Gradient evaluation took 2e-05 seconds
#> Chain 2: 1000 transitions using 10 leapfrog steps per transition would take 0.2 seconds.
#> Chain 2: Adjust your expectations accordingly!
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#> Chain 2: 
#> 
#> SAMPLING FOR MODEL 'anon_model' NOW (CHAIN 3).
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#> Chain 3: 
#> 
#> SAMPLING FOR MODEL 'anon_model' NOW (CHAIN 4).
#> Chain 4: 
#> Chain 4: Gradient evaluation took 2e-05 seconds
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#> Chain 4: 
summary(fit1)
#>  Family: gaussian 
#>   Links: mu = identity 
#> Formula: ls ~ mo(income) 
#>    Data: dat (Number of observations: 100) 
#>   Draws: 4 chains, each with iter = 2000; warmup = 1000; thin = 1;
#>          total post-warmup draws = 4000
#> 
#> Regression Coefficients:
#>           Estimate Est.Error l-95% CI u-95% CI Rhat Bulk_ESS Tail_ESS
#> Intercept    29.37      1.53    26.29    32.32 1.00     1988     2123
#> moincome     15.11      0.69    13.80    16.48 1.00     1904     2367
#> 
#> Monotonic Simplex Parameters:
#>              Estimate Est.Error l-95% CI u-95% CI Rhat Bulk_ESS Tail_ESS
#> moincome1[1]     0.64      0.04     0.56     0.72 1.00     3495     2518
#> moincome1[2]     0.28      0.05     0.19     0.37 1.00     2528     2166
#> moincome1[3]     0.08      0.04     0.01     0.16 1.00     2098     1417
#> 
#> Further Distributional Parameters:
#>       Estimate Est.Error l-95% CI u-95% CI Rhat Bulk_ESS Tail_ESS
#> sigma     7.07      0.52     6.15     8.18 1.00     2728     2287
#> 
#> Draws were sampled using sampling(NUTS). For each parameter, Bulk_ESS
#> and Tail_ESS are effective sample size measures, and Rhat is the potential
#> scale reduction factor on split chains (at convergence, Rhat = 1).
plot(fit1, N = 6)
#> Warning: Argument 'N' is deprecated. Please use argument 'nvariables' instead.

plot(conditional_effects(fit1), points = TRUE)


# model interaction with other variables
dat$x <- sample(c("a", "b", "c"), 100, TRUE)
fit2 <- brm(ls ~ mo(income)*x, data = dat)
#> Compiling Stan program...
#> Start sampling
#> 
#> SAMPLING FOR MODEL 'anon_model' NOW (CHAIN 1).
#> Chain 1: 
#> Chain 1: Gradient evaluation took 5.7e-05 seconds
#> Chain 1: 1000 transitions using 10 leapfrog steps per transition would take 0.57 seconds.
#> Chain 1: Adjust your expectations accordingly!
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#> Chain 1:                1.53 seconds (Total)
#> Chain 1: 
#> 
#> SAMPLING FOR MODEL 'anon_model' NOW (CHAIN 2).
#> Chain 2: 
#> Chain 2: Gradient evaluation took 4.4e-05 seconds
#> Chain 2: 1000 transitions using 10 leapfrog steps per transition would take 0.44 seconds.
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#> Chain 2: 
#> 
#> SAMPLING FOR MODEL 'anon_model' NOW (CHAIN 3).
#> Chain 3: 
#> Chain 3: Gradient evaluation took 4.4e-05 seconds
#> Chain 3: 1000 transitions using 10 leapfrog steps per transition would take 0.44 seconds.
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#> Chain 3: 
#> 
#> SAMPLING FOR MODEL 'anon_model' NOW (CHAIN 4).
#> Chain 4: 
#> Chain 4: Gradient evaluation took 4e-05 seconds
#> Chain 4: 1000 transitions using 10 leapfrog steps per transition would take 0.4 seconds.
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#> Chain 4:                1.417 seconds (Total)
#> Chain 4: 
summary(fit2)
#>  Family: gaussian 
#>   Links: mu = identity 
#> Formula: ls ~ mo(income) * x 
#>    Data: dat (Number of observations: 100) 
#>   Draws: 4 chains, each with iter = 2000; warmup = 1000; thin = 1;
#>          total post-warmup draws = 4000
#> 
#> Regression Coefficients:
#>             Estimate Est.Error l-95% CI u-95% CI Rhat Bulk_ESS Tail_ESS
#> Intercept      29.63      1.92    25.91    33.52 1.00     2480     2549
#> xb              1.37      2.86    -4.68     6.64 1.00     2161     2125
#> xc             -2.32      2.91    -8.42     3.26 1.00     2097     2311
#> moincome       15.62      0.98    13.74    17.58 1.00     1855     2361
#> moincome:xb    -1.22      1.53    -4.19     1.78 1.00     1889     2193
#> moincome:xc    -0.45      1.56    -3.58     2.68 1.00     1947     2692
#> 
#> Monotonic Simplex Parameters:
#>                 Estimate Est.Error l-95% CI u-95% CI Rhat Bulk_ESS Tail_ESS
#> moincome1[1]        0.63      0.04     0.55     0.71 1.00     2938     3122
#> moincome1[2]        0.29      0.04     0.20     0.38 1.00     3451     2668
#> moincome1[3]        0.08      0.04     0.01     0.17 1.00     2523     1689
#> moincome:xb1[1]     0.28      0.22     0.01     0.80 1.00     3157     2457
#> moincome:xb1[2]     0.32      0.23     0.01     0.82 1.00     4046     2264
#> moincome:xb1[3]     0.40      0.25     0.02     0.88 1.00     3075     2467
#> moincome:xc1[1]     0.33      0.23     0.01     0.83 1.00     3010     1859
#> moincome:xc1[2]     0.32      0.23     0.01     0.83 1.00     4072     2495
#> moincome:xc1[3]     0.35      0.24     0.01     0.84 1.00     4425     2642
#> 
#> Further Distributional Parameters:
#>       Estimate Est.Error l-95% CI u-95% CI Rhat Bulk_ESS Tail_ESS
#> sigma     7.03      0.53     6.05     8.17 1.00     3785     2812
#> 
#> Draws were sampled using sampling(NUTS). For each parameter, Bulk_ESS
#> and Tail_ESS are effective sample size measures, and Rhat is the potential
#> scale reduction factor on split chains (at convergence, Rhat = 1).
plot(conditional_effects(fit2), points = TRUE)




# ensure conditional monotonicity
fit3 <- brm(ls ~ mo(income, id = "i")*x, data = dat)
#> Compiling Stan program...
#> Start sampling
#> 
#> SAMPLING FOR MODEL 'anon_model' NOW (CHAIN 1).
#> Chain 1: 
#> Chain 1: Gradient evaluation took 5.3e-05 seconds
#> Chain 1: 1000 transitions using 10 leapfrog steps per transition would take 0.53 seconds.
#> Chain 1: Adjust your expectations accordingly!
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#> Chain 1: 
#> 
#> SAMPLING FOR MODEL 'anon_model' NOW (CHAIN 2).
#> Chain 2: 
#> Chain 2: Gradient evaluation took 4.6e-05 seconds
#> Chain 2: 1000 transitions using 10 leapfrog steps per transition would take 0.46 seconds.
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#> 
#> SAMPLING FOR MODEL 'anon_model' NOW (CHAIN 3).
#> Chain 3: 
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#> 
#> SAMPLING FOR MODEL 'anon_model' NOW (CHAIN 4).
#> Chain 4: 
#> Chain 4: Gradient evaluation took 4.4e-05 seconds
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#> Chain 4: 
summary(fit3)
#>  Family: gaussian 
#>   Links: mu = identity 
#> Formula: ls ~ mo(income, id = "i") * x 
#>    Data: dat (Number of observations: 100) 
#>   Draws: 4 chains, each with iter = 2000; warmup = 1000; thin = 1;
#>          total post-warmup draws = 4000
#> 
#> Regression Coefficients:
#>                  Estimate Est.Error l-95% CI u-95% CI Rhat Bulk_ESS Tail_ESS
#> Intercept           30.17      2.08    26.03    34.35 1.00     1996     2388
#> xb                   0.08      3.49    -7.00     6.88 1.00     1899     2163
#> xc                  -3.15      3.42    -9.68     3.43 1.00     1814     2331
#> moincomeidEQi       15.14      0.98    13.24    17.06 1.00     1641     1870
#> moincomeidEQi:xb    -0.14      1.54    -3.10     2.91 1.00     1724     2381
#> moincomeidEQi:xc     0.12      1.53    -2.83     3.15 1.00     1803     1895
#> 
#> Monotonic Simplex Parameters:
#>                      Estimate Est.Error l-95% CI u-95% CI Rhat Bulk_ESS
#> moincomeidEQi1[1]        0.65      0.04     0.57     0.72 1.00     3928
#> moincomeidEQi1[2]        0.28      0.04     0.20     0.37 1.00     2401
#> moincomeidEQi1[3]        0.07      0.04     0.01     0.15 1.00     2138
#> moincomeidEQi:xb1[1]     0.65      0.04     0.57     0.72 1.00     3928
#> moincomeidEQi:xb1[2]     0.28      0.04     0.20     0.37 1.00     2401
#> moincomeidEQi:xb1[3]     0.07      0.04     0.01     0.15 1.00     2138
#> moincomeidEQi:xc1[1]     0.65      0.04     0.57     0.72 1.00     3928
#> moincomeidEQi:xc1[2]     0.28      0.04     0.20     0.37 1.00     2401
#> moincomeidEQi:xc1[3]     0.07      0.04     0.01     0.15 1.00     2138
#>                      Tail_ESS
#> moincomeidEQi1[1]        3064
#> moincomeidEQi1[2]        2373
#> moincomeidEQi1[3]        1174
#> moincomeidEQi:xb1[1]     3064
#> moincomeidEQi:xb1[2]     2373
#> moincomeidEQi:xb1[3]     1174
#> moincomeidEQi:xc1[1]     3064
#> moincomeidEQi:xc1[2]     2373
#> moincomeidEQi:xc1[3]     1174
#> 
#> Further Distributional Parameters:
#>       Estimate Est.Error l-95% CI u-95% CI Rhat Bulk_ESS Tail_ESS
#> sigma     7.07      0.54     6.12     8.23 1.00     3412     2629
#> 
#> Draws were sampled using sampling(NUTS). For each parameter, Bulk_ESS
#> and Tail_ESS are effective sample size measures, and Rhat is the potential
#> scale reduction factor on split chains (at convergence, Rhat = 1).
plot(conditional_effects(fit3), points = TRUE)



# }