# Chapter 5 Group Comparisons

Last week we discussed the binomial and the Poisson model, both of which are models with only one parameters. In most research, however, there are at least two parameters, and sometimes one is talking about tens or hundreds of parameters. For this week we will be looking at group comparisons done in a Bayesian way. We will start with between-group comparisons, extend it to multiple groups, and then move on to comparisons for repeated measures.

## 5.1 Data

We will use the data in Frank, Biberci, and Verschuere (2019), with is a replication study examining the response time (RT) for lying and whether RT is shorter or longer when lying in native language. In the original study, the researchers found that the difference between telling a lie and a truth (lie-truth difference) is smaller in English than in German for native German speakers. In the replication study, the same design was used to see whether the findings can be replicated on Dutch speakers. The data can be found at https://osf.io/x4rfk/.

Instead of looking at lie-truth difference across languages, we will start off by comparing difference in RT for telling a lie in Dutch between males (man) and females (vrouw), as the model is easier to understand for independent sample comparisons, and the model can be applied generally for between-subject experimental designs.

lies <- readxl::read_excel("../data/ENDFILE.xlsx")
# Rescale response time from ms to sec
lies <- lies %>%
mutate_at(vars(LDMRT:TEMRT), ~ . / 1000)
# Describe the data
psych::describe(lies %>% select(Age, LDMRT:TEMRT))
>#       vars  n  mean   sd median trimmed  mad   min   max range skew kurtosis
># Age      1 66 23.15 7.18  21.00   21.61 2.97 18.00 61.00 43.00 2.98    10.79
># LDMRT    2 63  1.47 0.51   1.41    1.42 0.47  0.53  3.26  2.73 0.97     1.41
># TDMRT    3 63  1.16 0.43   1.09    1.10 0.35  0.51  2.94  2.43 1.54     3.27
># LEMRT    4 63  1.45 0.46   1.41    1.43 0.50  0.49  2.42  1.93 0.29    -0.80
># TEMRT    5 63  1.29 0.41   1.28    1.26 0.42  0.57  2.60  2.02 0.86     0.69
>#         se
># Age   0.88
># LDMRT 0.06
># TDMRT 0.05
># LEMRT 0.06
># TEMRT 0.05
# Plot the data
psych::pairs.panels(lies %>%
select(Age, Gender, LDMRT:TEMRT))

## 5.2 Between-Subject Comparisons

### 5.2.1 Plots

lies %>%
select(PP, Gender, LDMRT, TDMRT) %>%
gather(key = "veracity", value = "RT", LDMRT:TDMRT) %>%
ggplot(aes(x = RT, col = veracity)) +
geom_density(bw = "SJ") +
facet_wrap(~ Gender)
># Warning: Removed 6 rows containing non-finite values (stat_density).

### 5.2.2 Independent sample t-test

# independent t-test
t.test(lies$LDMRT[which(lies$Gender == "man")],
lies$LDMRT[which(lies$Gender == "vrouw")])
>#
>#  Welch Two Sample t-test
>#
># data:  lies$LDMRT[which(lies$Gender == "man")] and lies$LDMRT[which(lies$Gender == "vrouw")]
># t = 4, df = 34, p-value = 3e-04
># alternative hypothesis: true difference in means is not equal to 0
># 95 percent confidence interval:
>#  0.255 0.778
># sample estimates:
># mean of x mean of y
>#      1.81      1.30

### 5.2.3 Bayesian Normal Model

The $$t$$-test above does not directly show the underlying model. In fully Bayesian analyses, this has to be made explicit. A statistical model states the underlying assumptions of the statistical procedure. And to say more directly, a model is mostly a set of distributional assumptions. Below is the one for the $$t$$-test, including some prior choices.

\begin{align} \texttt{LDMRT}_{ij} & \sim \mathcal{N}^+(\mu_j, \sigma) \\ \mu_j & = \begin{cases} \mu_1, & j = 1 \\ \mu_1 + \beta, & j = 2 \end{cases} \\ \mu_1 & \sim \mathcal{N}^+(0.5, 2.5) \\ \beta & \sim \mathcal{N}(0, 2.5) \\ \sigma & \sim t^+(4, 0, 1) \end{align}

Parameters:

• $$\mu_j$$: mean parameter for the $$j$$th group where $$j$$ = 1 for man and $$j$$ = 2 for vrouw
• $$\sigma$$: standard deviation parameter that’s assumed the same across the two gender groups
• $$\beta$$: mean difference between the two genders

Transformed parameters:

• $$\mu_2$$

The above prior distributions are called weakly informative. They are informative because it does give some informative on what the most likely value is for each parameter prior to looking at the data, such as 0.5 for $$\mu_1$$. On the other hand, it is weak because the most likely values are basically covering all values that are reasonable. For example, a normal prior with a mean 0 and an SD of 2.5 for $$\beta$$ represents a belief that the difference in RT between males and females has a 68% chance to be no more than 2.5 seconds. Given that most people took just no more than two seconds to respond, this is a pretty weak prior.

For $$\sigma$$, its lower bound is 0. The prior used here is a half-$$t$$ distribution based on Gelman (2006). You may find this guide: https://github.com/stan-dev/stan/wiki/Prior-Choice-Recommendations to be useful when choosing priors for a lot of the models we will discuss in this course.

Below are the graphs for the three priors:

p1 <- ggplot(tibble(mu = c(0, 5)), aes(x = mu)) +
stat_function(fun = function(x) dnorm(x, mean = 0.5, sd = 2.5) /
(1 - pnorm(0, 0.5, sd = 2.5))) +
labs(x = expression(mu), y = "Density")
p2 <- ggplot(tibble(beta = c(-5, 5)), aes(x = beta)) +
stat_function(fun = dnorm, args = list(mean = 0, sd = 2.5)) +
labs(x = expression(beta), y = "Density")
p3 <- ggplot(tibble(sig = c(0, 5)), aes(x = sig)) +
stat_function(fun = function(x) dt(x, df = 2) * 2) +
labs(x = expression(sigma), y = "Density")
gridExtra::grid.arrange(p1, p2, p3, nrow = 2)

#### 5.2.3.2 STAN

Below is the STAN code for the model:

data {
int<lower=0> N1;  // number of observations (group 1)
int<lower=0> N2;  // number of observations (group 2)
vector[N1] y1;  // response time (group 1);
vector[N2] y2;  // response time (group 2);
}
parameters {
real<lower=0> mu_1;  // mean of group 1
real beta;  // difference in means
real<lower=0> sigma;  // pooled standard deviation
}
transformed parameters {
real<lower=0> mu_2 = mu_1 + beta;
}
model {
y1 ~ normal(mu_1, sigma);
y2 ~ normal(mu_2, sigma);
// prior
mu_1 ~ normal(0.5, 2.5);
beta ~ normal(0, 2.5);
sigma ~ student_t(4, 0, 1);
}
generated quantities {
real y1rep[N1];
real y2rep[N2];
for (i in 1:N1) {
y1rep[i] = normal_rng(mu_1, sigma);
}
for (i in 1:N2) {
y2rep[i] = normal_rng(mu_2, sigma);
}
}

And let’s fit the model with rstan.

library(rstan)
rstan_options(auto_write = TRUE)
# Exclude missing values
lies_cc <- drop_na(lies, LDMRT)
m1 <- stan("../codes/group_comparison.stan",
data = list(N1 = sum(lies_cc$Gender == "man"), N2 = sum(lies_cc$Gender == "vrouw"),
y1 = lies_cc$LDMRT[which(lies_cc$Gender == "man")],
y2 = lies_cc$LDMRT[which(lies_cc$Gender == "vrouw")]))
# Use the broom package to generate nicely formatted table
broom::tidy(m1, pars = c("mu_1", "mu_2", "beta", "sigma"),
conf.int = TRUE, conf.method = "quantile")
># # A tibble: 4 x 5
>#   term  estimate std.error conf.low conf.high
>#   <chr>    <dbl>     <dbl>    <dbl>     <dbl>
># 1 mu_1     1.81     0.0999    1.61      2.00
># 2 mu_2     1.30     0.0724    1.15      1.44
># 3 beta    -0.511    0.125    -0.759    -0.265
># 4 sigma    0.463    0.0427    0.388     0.557
plot(m1, pars = c("mu_1", "mu_2", "beta", "sigma"))
># ci_level: 0.8 (80% intervals)
># outer_level: 0.95 (95% intervals)

#### 5.2.3.3 Effect size

For mean comparisons, it is also common to report a standardized effect size. For example, Cohen’s $$d$$ was defined as: $d = \frac{\mu_2 - \mu_1}{\sigma}$

We can directly put that in the STAN code for generated quantities, such as

generated quantities {
real cohen_d = (mu_2 - mu_1) / sigma;
}

However, if you have already run STAN, like what we did here, we can easily compute the effect size using the posterior samples of beta and sigma, as explained in the code below:

# Extract posterior samples of the beta and the sigma parameters
post_sam <- as.data.frame(m1, pars = c("beta", "sigma"))
# Compute Cohen's d for each iteration
N2 = sum(lies_cc$Gender == "vrouw"), y1 = lies_cc$LDMRT[which(lies_cc$Gender == "man")], y2 = lies_cc$LDMRT[which(lies_cc$Gender == "vrouw")]), control = list(adapt_delta = .999, max_treedepth = 12)) broom::tidy(m3, pars = c("mu_1", "mu_2", "beta_mu", "sigma", "ndt_1", "ndt_2"), conf.int = TRUE, conf.method = "HPDinterval") %>% # Just to demonstrate HPDI knitr::kable(digits = 2) # round to two digits term estimate std.error conf.low conf.high mu_1 0.15 0.21 -0.25 0.54 mu_2 0.03 0.12 -0.21 0.24 beta_mu -0.12 0.19 -0.48 0.26 sigma 0.40 0.06 0.30 0.52 ndt_1 0.54 0.21 0.14 0.91 ndt_2 0.18 0.09 0.02 0.36 #### 5.2.5.3 Difference in $$\mu$$ and in ndt # Extract posterior samples of the mu, sigma, and ndt parameters post_sam <- as.data.frame(m3, pars = c("mu_1", "mu_2", "sigma", "ndt_1", "ndt_2")) # Compute means of decision components for each iteration post_sam$dt_1 <- exp(post_sam$mu_1 + post_sam$sigma^2 / 2)
post_sam$dt_2 <- exp(post_sam$mu_2 + post_sam$sigma^2 / 2) post_sam$dt_diff <- post_sam$dt_2 - post_sam$dt_1
# Compute difference for ndt
y2 = lies_cc$LDMRT), pars = "zu", include = FALSE, seed = 104134) You might receive a warning from rstan from the above code. Don’t ignore those. Generally, when STAN shows a warning it usually indicates that the results are not trustworthy. #### 5.4.3.2 Effect size For repeated measures, there were several different ways to define effect size. To make it consistent with the between-subject comparison, Cohen’s $$d$$ was defined as: $d = \frac{\mu_2 - \mu_1}{\sqrt{\sigma^2 + \tau^2}}$ This time we directly put that in the STAN code for generated quantities. generated quantities { real cohen_d = (mu_2 - mu_1) / sqrt(sigma^2 + tau^2); } # Use the broom package to generate nicely formatted table broom::tidy(m4, pars = c("mu_1", "mu_2", "beta", "sigma", "tau", "cohen_d"), conf.int = TRUE, conf.method = "quantile") ># # A tibble: 6 x 5 ># term estimate std.error conf.low conf.high ># <chr> <dbl> <dbl> <dbl> <dbl> ># 1 mu_1 1.16 0.0604 1.04 1.28 ># 2 mu_2 1.47 0.0606 1.35 1.59 ># 3 beta 0.308 0.0346 0.240 0.377 ># 4 sigma 0.194 0.0177 0.163 0.233 ># 5 tau 0.441 0.0453 0.364 0.539 ># 6 cohen_d 0.644 0.0906 0.471 0.828 plot(m4, pars = c("mu_1", "mu_2", "beta", "sigma", "tau", "cohen_d")) ># ci_level: 0.8 (80% intervals) ># outer_level: 0.95 (95% intervals) From the normal model, it was estimated that the mean RT for truth was 1.159 seconds, 95% CI [1.040, 1.279]. On average women had faster RT when asked to tell lies in Dutch than man, with an estimated difference of 0.308 seconds, 95% CI [0.240, 0.377], d = 0.644, 95% CI [0.471, 0.828]. #### 5.4.3.3 Posterior Predictive Check Let’s check whether the model works well. First look at the shape of the data: library(bayesplot) # Observed data y1 <- lies_cc$TDMRT
y2 <- lies_cc\$LDMRT
# Replicated data (randomly sampling 100)
y1rep <- as.matrix(m4, pars = "y1rep")[sample.int(2000, 100), ]
y2rep <- as.matrix(m4, pars = "y2rep")[sample.int(2000, 100), ]
ppc_dens_overlay(y1, yrep = y1rep)

ppc_dens_overlay(y2, yrep = y2rep)

The fit was good for lies but not for truth. Below is a check for outliers:

ppc_intervals(y1, yrep = y1rep)

ppc_intervals(y2, yrep = y2rep)

As discussed in the previous note, you may want to consider some alternative models, maybe the Student’s $$t$$ likelihood that accommodate outliers, or some response time models like the shifted lognormal model. I encourage you to modify the STAN code and try things out!

### 5.4.4 Using brms*

Finally, one thing to mention is that many of the commonly used models have been implemented in the R package brms. I decided to talk about STAN first because it is the underlying engine and can fit almost any parametric models, but in practice I do use brms a lot. I will show you the brms code for a majority of the models moving forward. For example, here is one for the repeated measure comparison above, with a Student’s $$t$$ likelihood. It does require restructuring the data first.

library(brms)
lies_long <- lies %>%
select(PP, Gender, LDMRT, TDMRT) %>%
gather(key = "veracity", value = "RT", LDMRT:TDMRT)
m2_brm <- brm(RT ~ veracity + (1 | PP), data = lies_long,
family = student(),
prior = c(prior(normal(0, 1), class = "b"),
prior(student_t(4, 0, 2.5), class = "sd"),
prior(student_t(4, 0, 2.5), class = "sigma")))
># Warning: Rows containing NAs were excluded from the model.
># Warning: Bulk Effective Samples Size (ESS) is too low, indicating posterior means and medians may be unreliable.
># Running the chains for more iterations may help. See
># http://mc-stan.org/misc/warnings.html#bulk-ess

And there are some nice functions you can use to summarize models fitted by brms

m2_brm
>#  Family: student
>#   Links: mu = identity; sigma = identity; nu = identity
># Formula: RT ~ veracity + (1 | PP)
>#    Data: lies_long (Number of observations: 126)
># Samples: 4 chains, each with iter = 2000; warmup = 1000; thin = 1;
>#          total post-warmup samples = 4000
>#
># Group-Level Effects:
># ~PP (Number of levels: 63)
>#               Estimate Est.Error l-95% CI u-95% CI Rhat Bulk_ESS Tail_ESS
># sd(Intercept)     0.41      0.04     0.34     0.50 1.01      732     1417
>#
># Population-Level Effects:
>#               Estimate Est.Error l-95% CI u-95% CI Rhat Bulk_ESS Tail_ESS
># Intercept         1.45      0.06     1.33     1.56 1.01      361      801
># veracityTDMRT    -0.30      0.03    -0.36    -0.24 1.00     4341     2787
>#
># Family Specific Parameters:
>#       Estimate Est.Error l-95% CI u-95% CI Rhat Bulk_ESS Tail_ESS
># sigma     0.14      0.02     0.10     0.19 1.00     1110     1694
># nu        5.75      4.77     2.08    19.49 1.00     1603     1783
>#
># Samples were drawn 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).
# sjPlot::tab_model(m2_brm, show.ci50 = FALSE)  # broken in newest version of brms
plot(m2_brm)

From above, as the estimate of $$\nu$$ was quite small, it was pretty clear that there are outliers that need to be handled (and was handled somewhat by the robust Student’s $$t$$ model). Let’s look at some posterior predictive checks:

pp_check(m2_brm, nsamples = 100)

pp_check(m2_brm, type = "intervals_grouped", group = "veracity")
># Using all posterior samples for ppc type 'intervals_grouped' by default.

### 5.4.5 Region of Practical Equivalence (ROPE)

One thing that is often of interest in research is to establish equivalence between two values. For example, we may wonder, for example,

• whether an experimental manipulation has an effect size of zero ($$d$$ = 0),
• whether two variables are truly uncorrelated ($$r$$ = 0),
• whether the blood type distribution is the same across countires (see some examples here)
• whether two parallel forms of a test have the same difficulty level

In this example, we can investigate, for example,

Whether lying in native and in second languages requires the same response time.

In all these above scenarios, we are interested in “confirming” whether a quantity is equal to another quantity. The traditional null hypothesis significance testing (NHST), however, won’t allow us to do that. That’s because NHST is set up to reject the null hypothesis, but failure to reject $$d$$ = 0 does not confirm that $$d$$ = 0; it just means that we don’t have enough evidence for that. In addition, we know in advance, with high degree of certainty, that $$d$$ $$\neq$$ 0. Do we truly believe that the treatment group and the control group will perform exactly the same on an IQ test? Even when one group got 100 and the other group 100.0000001, the null hypothesis is false.

Therefore, what we really meant when saying whether two things are equal is not that whether two quantities are exactly the same, which is basically impossible, but instead whether two quantities are close enough, or practically equivalent. In the IQ test example, most of us would agree that the two groups are practically equivalent.

So what we actually want to test, in mathematically notation, is $|\beta - \beta_0| < \epsilon,$ where $$\beta$$ is the parameter of interest (in this case the difference in RT between lying in Dutch and lying in English), $$\beta_0$$ is the value we wanted to compare $$\beta$$ to (in this case 0), and $$\epsilon$$ is some small value of a difference by which $$\beta$$ and $$\beta_0$$ are deemed practically equivalent. For example, in our analysis we may think that if the difference is less than .05 seconds (or 50ms), we may say that there are no difference in RT. In other words, if there is a high probability (in a Bayesian sense) that $-\epsilon < \beta < \epsilon$ than we considered there is sufficient evident that $$\beta$$ = $$\beta_0$$ in a practical sense. The interval ($$-\epsilon$$, $$\epsilon$$) is what Kruschke (2013) referred to as the region of practical equivalence, or ROPE.

Using brms, we run a model comparing the means of LDMRT and LEMRT:

library(brms)
lies_long <- lies %>%
select(PP, Gender, LDMRT, LEMRT) %>%
gather(key = "language", value = "RT", LDMRT, LEMRT)
m3_brm <- brm(RT ~ language + (1 | PP), data = lies_long,
family = student(),
prior = c(prior(normal(0, 1), class = "b"),
prior(student_t(4, 0, 2.5), class = "sd"),
prior(student_t(4, 0, 2.5), class = "sigma")),
iter = 4000)
># Warning: Rows containing NAs were excluded from the model.

As you can see, the 95% CI of $$\beta$$ is

brms::posterior_interval(m3_brm, pars = "b_languageLEMRT")
>#                    2.5%  97.5%
># b_languageLEMRT -0.0405 0.0399

So we can see that the 95% CI is completely inside the ROPE. Therefore, we would say:

There is > 95% chance that the mean RT of lying is practically the same in Dutch and in English

You can also check the probability that $$\beta$$ is inside the ROPE:

# Extract posterior samples
beta_sam <- as.matrix(m3_brm, pars = "b_languageLEMRT")
# Probability in the ROPE
mean(beta_sam < .05 & beta_sam > -.05)
># [1] 0.982

which shows that there is a mean(beta_sam < .05 & beta_sam > -.05) * 100% chance that the mean RT of lying is practically the same in Dutch and in English.

You can do the same in frequentist with equivalence testing (see https://daniellakens.blogspot.com/2017/02/rope-and-equivalence-testing.html), but I found ROPE to be more scalable to other types of models, plus you’ll obtain a posterior interval anyway with Bayesian analyses.

### References

Frank, Avi, Sena Biberci, and Bruno Verschuere. 2019. “The language of lies: a preregistered direct replication of Suchotzki and Gamer (2018; Experiment 2).” Cognition and Emotion 33 (6): 1310–5. https://doi.org/10.1080/02699931.2018.1553148.

Gelman, Andrew. 2006. “Prior distributions for variance parameters in hierarchical models (Comment on Article by Browne and Draper).” Bayesian Analysis 1 (3): 515–34. https://doi.org/10.1214/06-BA117A.

Heathcote, Andrew, Scott Brown, and Denis Cousineau. 2004. “QMPE: Estimating Lognormal, Wald, and Weibull Rt Distributions with a Parameter-Dependent Lower Bound.” Behavior Research Methods, Instruments, & Computers 36 (2): 277–90.

Kruschke, John K. 2013. “Bayesian estimation supersedes the t test.” Journal of Experimental Psychology: General 142 (2): 573–603. https://doi.org/10.1037/a0029146.