# Interpreting/reporting BRMS result for nominal variables with two factors(Linguistics Data)

Greetings,

I am relatively new to Bayesian stuff and have some questions (beginner’s question).
Here is the data I am working on. All variables, except two are nominal/categorical data. The idea is to assess effects of individual variable(predictors) on the outcome(Alternation, categorical data with two factors) with random effects(Native_Language)

``````data.frame':	3485 obs. of  12 variables:
\$ Native_Language  : Factor w/ 27 levels "Bulgarian","Chinese",..: 1 1 1 1 1 1 1 1 1 1 ...
\$ Agent_Pos        : Factor w/ 3 levels "NOUN","PRON",..: 2 1 2 2 1 3 3 3 3 3 ...
\$ Agent_Animacy    : Factor w/ 2 levels "Animate","Inanimate": 1 1 1 1 2 2 2 2 2 2 ...
\$ Verb             : Factor w/ 194 levels "absorb","accuse",..: 161 77 77 77 22 11 77 77 77 77 ...
\$ Semantic_Class   : Factor w/ 6 levels "a","c","f","nd",..: 1 1 1 1 1 3 1 1 1 1 ...
\$ Theme_Pos        : Factor w/ 3 levels "NOUN","PRON",..: 3 3 2 2 2 1 1 1 1 1 ...
\$ Theme_Animacy    : Factor w/ 2 levels "Animate","Inanimate": 2 2 2 2 1 2 2 2 2 2 ...
\$ Theme_length     : num  0.699 0.845 0.903 0.301 0.477 ...
\$ Recipient_Pos    : Factor w/ 3 levels "NOUN","PRON",..: 2 2 2 1 2 2 3 2 2 2 ...
\$ Recipient_Animacy: Factor w/ 2 levels "Animate","Inanimate": 2 1 1 2 1 1 2 1 1 1 ...
\$ Recipient_length : num  0.301 0.301 0.602 0.477 0.477 ...
\$ Alternation      : Factor w/ 2 levels "Double Object",..: 1 1 1 2 2 1 2 1 1 1 ...
``````

Here is my model with weak prior informants

`alternation_model <- brm(Alternation ~ (1 | Native_Language) + Agent_Pos + Agent_Animacy + Semantic_Class + Theme_Pos + Theme_Animacy + Theme_length + Recipient_Pos + Recipient_Animacy + Recipient_length, data = df, warmup = 1000, iter = 3000, cores = 4, chains = 4, family = bernoulli(link = 'logit'))`

and the results are as follows;

``````Family: bernoulli
Formula: Alternation ~ (1 | Native_Language) + Agent_Pos + Agent_Animacy + Semantic_Class + Theme_Pos + Theme_Animacy + Theme_length + Recipient_Pos + Recipient_Animacy + Recipient_length
Data: df (Number of observations: 3485)
Draws: 4 chains, each with iter = 3000; warmup = 1000; thin = 1;
total post-warmup draws = 8000

Group-Level Effects:
~Native_Language (Number of levels: 27)
Estimate Est.Error l-95% CI u-95% CI Rhat Bulk_ESS Tail_ESS
sd(Intercept)     0.29      0.07     0.16     0.44 1.00     3318     5461

Population-Level Effects:
Estimate Est.Error l-95% CI u-95% CI Rhat Bulk_ESS Tail_ESS
Intercept                     -1.20      0.41    -2.02    -0.40 1.00    14616     6076
Agent_PosPRON                 -0.12      0.11    -0.33     0.09 1.00    13552     6037
Agent_PosPROPN                 0.06      0.18    -0.31     0.42 1.00    15941     6282
Agent_AnimacyInanimate        -0.40      0.10    -0.61    -0.19 1.00    13728     6330
Semantic_Classc               -0.88      0.22    -1.32    -0.44 1.00    16805     5672
Semantic_Classf                0.51      0.17     0.17     0.85 1.00    16874     5879
Semantic_Classnd              -1.85      0.32    -2.49    -1.26 1.00    15973     6077
Semantic_Classp               -2.76      0.43    -3.68    -1.98 1.00    16276     5585
Semantic_Classt                0.45      0.16     0.14     0.75 1.00    15594     5528
Theme_PosPRON                  1.25      0.22     0.83     1.68 1.00    16202     5617
Theme_PosPROPN                 0.43      0.51    -0.62     1.40 1.00    16901     5790
Theme_AnimacyInanimate         0.60      0.27     0.09     1.13 1.00    16548     5931
Theme_length                  -1.16      0.24    -1.64    -0.69 1.00    16606     6162
Recipient_PosPRON             -1.48      0.13    -1.74    -1.23 1.00    12939     6610
Recipient_PosPROPN             0.02      0.27    -0.50     0.55 1.00    17995     6193
Recipient_AnimacyInanimate     0.62      0.11     0.39     0.84 1.00    15439     6132
Recipient_length               2.31      0.29     1.75     2.86 1.00    12869     6375

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).
``````

1)First question is how to interpret results? Particularly negative estimate scores(I assume these are median scores for centrality). Does the scores for negatives mean that data is left-skewed? or Are they simply correlation results i.e predictors and outcomes are negatively correlated? If so, should I say that, for instance, likelihood of `Semantic_Class` being p(semantic classification labeled as `p`) reduces the likelihood of outcome being Double Object(codify as 1 in factor data)?

1. How about the effect of random effects (Native Language)? Group-Level Effects result is as;

sd(Intercept) 0.29 0.07 0.16 0.44 1.00 3318 5461

However, I got the results for individual 27 factors from `describe_posterior(results, test = c("p_direction", "rope", "bayesfactor", "equivalence_test", "p_map"), effects = 'random')`

``````Parameter                         |    Median |         95% CI | p (MAP) |     pd |          ROPE | % in ROPE | Equivalence (ROPE) |  Rhat |     ESS |    BF
------------------------------------------------------------------------------------------------------------------------------------------------------------
Native_Language:Bulgarian         |      0.16 | [-0.16,  0.52] |  0.711  | 83.58% | [-0.18, 0.18] |    54.75% |          Undecided | 0.999 | 5583.00 |  1.28
Native_Language:Chinese           |     -0.05 | [-0.45,  0.38] |  0.956  | 59.05% | [-0.18, 0.18] |    66.01% |          Undecided | 1.000 | 6049.00 | 0.789
Native_Language:Chinese-Cantonese |      0.33 | [ 0.08,  0.62] |  0.067  | 99.28% | [-0.18, 0.18] |    10.92% |          Undecided | 0.999 | 3733.00 | 11.57
Native_Language:Czech             |      0.32 | [-0.07,  0.72] |  0.287  | 95.15% | [-0.18, 0.18] |    23.81% |          Undecided | 1.000 | 4753.00 |  3.71
Native_Language:Dutch             |     -0.05 | [-0.45,  0.33] |  0.970  | 61.05% | [-0.18, 0.18] |    66.38% |          Undecided | 1.000 | 5849.00 | 0.777
Native_Language:Finnish           |      0.22 | [-0.16,  0.60] |  0.541  | 87.75% | [-0.18, 0.18] |    43.07% |          Undecided | 1.000 | 4672.00 |  1.65
Native_Language:French            |     -0.29 | [-0.72,  0.13] |  0.429  | 92.22% | [-0.18, 0.18] |    30.76% |          Undecided | 1.000 | 4848.00 |  2.43
Native_Language:German            |      0.05 | [-0.27,  0.41] |  0.939  | 62.68% | [-0.18, 0.18] |    72.17% |          Undecided | 1.000 | 6180.00 | 0.743
Native_Language:Greek             |     -0.26 | [-0.64,  0.08] |  0.376  | 93.08% | [-0.18, 0.18] |    31.62% |          Undecided | 1.000 | 4868.00 |  2.50
Native_Language:Hungarian         |     -0.25 | [-0.59,  0.07] |  0.325  | 93.95% | [-0.18, 0.18] |    33.31% |          Undecided | 1.000 | 4682.00 |  2.37
Native_Language:Italian           |      0.30 | [-0.04,  0.70] |  0.276  | 95.60% | [-0.18, 0.18] |    24.20% |          Undecided | 1.000 | 5318.00 |  2.86
Native_Language:Japanese          |     -0.18 | [-0.54,  0.14] |  0.645  | 85.12% | [-0.18, 0.18] |    49.86% |          Undecided | 0.999 | 6196.00 |  1.29
Native_Language:Korean            |     -0.02 | [-0.37,  0.31] |  0.976  | 53.95% | [-0.18, 0.18] |    74.19% |          Undecided | 1.000 | 5629.00 | 0.820
Native_Language:Lithuanian        |     -0.03 | [-0.40,  0.34] |  0.984  | 55.53% | [-0.18, 0.18] |    70.14% |          Undecided | 0.999 | 5587.00 | 0.922
Native_Language:Macedonian        |     -0.38 | [-0.78,  0.03] |  0.206  | 97.22% | [-0.18, 0.18] |    16.47% |          Undecided | 1.000 | 5201.00 |  5.61
Native_Language:Norwegian         |      0.02 | [-0.38,  0.40] |  0.975  | 54.47% | [-0.18, 0.18] |    69.24% |          Undecided | 1.000 | 6209.00 | 0.775
Native_Language:Persian           |     -0.23 | [-0.63,  0.09] |  0.475  | 91.10% | [-0.18, 0.18] |    37.60% |          Undecided | 1.000 | 5291.00 |  1.91
Native_Language:Polish            |     -0.19 | [-0.56,  0.16] |  0.598  | 87.08% | [-0.18, 0.18] |    47.65% |          Undecided | 0.999 | 4846.00 |  1.60
Native_Language:Portuguese        |      0.24 | [-0.11,  0.58] |  0.427  | 91.80% | [-0.18, 0.18] |    38.25% |          Undecided | 1.000 | 5300.00 |  2.12
Native_Language:Punjabi           | -1.35e-03 | [-0.47,  0.50] |  > .999 | 50.25% | [-0.18, 0.18] |    58.38% |          Undecided | 1.000 | 7047.00 | 0.847
Native_Language:Russian           |      0.01 | [-0.37,  0.38] |  0.993  | 53.62% | [-0.18, 0.18] |    71.27% |          Undecided | 0.999 | 7402.00 | 0.746
Native_Language:Serbian           |      0.06 | [-0.29,  0.42] |  0.947  | 62.75% | [-0.18, 0.18] |    69.72% |          Undecided | 1.000 | 6268.00 | 0.778
Native_Language:Spanish           |      0.22 | [-0.13,  0.63] |  0.547  | 88.62% | [-0.18, 0.18] |    42.65% |          Undecided | 0.999 | 4609.00 |  1.57
Native_Language:Swedish           |      0.19 | [-0.14,  0.53] |  0.594  | 86.35% | [-0.18, 0.18] |    48.75% |          Undecided | 1.000 | 5871.00 |  1.40
Native_Language:Tswana            |     -0.13 | [-0.47,  0.21] |  0.780  | 77.15% | [-0.18, 0.18] |    60.83% |          Undecided | 0.999 | 5093.00 | 0.921
Native_Language:Turkish           |      0.02 | [-0.38,  0.41] |  0.998  | 52.65% | [-0.18, 0.18] |    68.38% |          Undecided | 0.999 | 7183.00 | 0.791
Native_Language:Urdu              |     -0.07 | [-0.55,  0.36] |  0.975  | 63.15% | [-0.18, 0.18] |    58.70% |          Undecided | 0.999 | 7198.00 | 0.953
``````

How should I interpret comparing Native Languages as Group-Level Effect and individual levels?
Edit an additional question. For factor coding, first level ‘Double Object’ is coded as 1 and ‘Prepositional’ is coded as 2. Since brms/rstanarm creates dummy coding I assume levels are then coded as 0-1 in which 1 = Double Object 0 = Not Double Object(i.e Prepositional). Therefore, results represent Double Object. So how can I get regression results for Prepositional without re-running the model?

Finally, is there a commonly agreed guideline for reporting Bayes Regression analysis?

I got the basic idea of describing posterior distrubiton (following explanations provided here yet I cannot the estimate the direction of correlation.

Since you are new to Bayes and brms, my general recommendation would be to start with a much simpler model. Given your current use case, we can still clarify some of your questions.

1. When your predictor is a nominal variable (factor), R will use one of the categories as the reference category (the first level of a factor) and the other levels will be expressed as if they were dummy variables. Thus, the coefficients for the second category in a factor will be the deviation from that category, relative to the expected value of the first category. Since these are deviations, negative values are decreases relative to the reference category. Since you have used logistic regression, these are all on the log-odds scale.

2. Honestly, I wouldn’t even try to interpret the random effects until you have the fixed effects 100% nailed down. Very broadly speaking, the `sd(Intercept)` line in your summary output indicates the standard deviation for the distribution of intercepts across your different levels of Native_Language. Since I’m not a domain expert, I can’t tell you if that looks like it’s large or not. But in my world, a 0.29 standard deviation in coefficients on the log-odds scale is typically somewhere in the small to moderate range.

3. No, I don’t there’s a commonly agreed upon method of reporting a Bayesian analysis. But then again, there aren’t really etched-in-stone standards for reporting a frequentist analysis, either. Generally speaking, use the old “more information is better” standard for your working drafts and whittle down from there. Another approach is to go into great detail in your supplemental materials and simplify them in the primary manuscript. Also consider looking up a few papers from more experienced brms users and see what they do.

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Thank you so much for all the info. All clear now.

Actually, `Native_Language` represents data source for each sentence extracted from individual documents. In other words, learners’ first languages. Since apart from fixed effects, there might be some native language related factors which are not observed here, `Native_Language` is listed as random effects. I guess I am right with the idea.

`sd` is really small yet regarding second language teaching, having no or little effect might be still influential, to say, given all the predictors, no difference was observed thus there might be something else to consider, or regarding the distinctiveness of the data, if there are still similarites/a certain pattern in the outcome, there is a central tendency due to nature of the outcome etc…

I do read alot recently for bayesian analysis. Indeed it is a more natural approach for my field where no certanities are expected. Have any suggested for reading books/articles/blogs etc? It would really help as I feel like I’ve already consumed everything available online!

I’m partial to the first edition of Statistical Rethinking (Statistical Rethinking | Richard McElreath). For a brms translation of the text, you might check out Statistical rethinking with brms, ggplot2, and the tidyverse. You can find other resources from me at https://solomonkurz.netlify.app/.

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