13.4 Testing and probing moderation of mediation

13.4.1 A test of moderation of the relative indirect effect.

To get the slopes of Protest vs. No Protest and Collective vs. Individual Protest (i.e., \(a_{4}b\) and \(a_{5}b\), respectively), we just work directly with the posterior_samples(), which we’ve saved as post.

post <-
  post %>% 
  mutate(a4b = `b_respappr_D1:sexism`*b_liking_respappr,
         a5b = `b_respappr_D2:sexism`*b_liking_respappr) 

post %>% 
  select(a4b:a5b) %>%
  gather() %>% 
  group_by(key) %>% 
  summarize(mean = mean(value),
            ll = quantile(value, probs = .025),
            ul = quantile(value, probs = .975)) %>% 
  mutate_if(is.double, round, digits = 3)
## # A tibble: 2 x 4
##   key     mean     ll    ul
##   <chr>  <dbl>  <dbl> <dbl>
## 1 a4b    0.307  0.099  0.56
## 2 a5b   -0.089 -0.326  0.14

Here they are in a geom_halfeyeh() plot.

post %>% 
  select(a4b:a5b) %>%
  gather() %>% 
 
  ggplot(aes(x = value, y = key)) +
  geom_halfeyeh(point_interval = median_qi, .prob = c(0.95, 0.5),
                fill = "grey50", color = "white") +
  scale_y_discrete(expand = c(.1, .1)) +
  labs(x = NULL, y = NULL) +
  theme_black() +
  theme(axis.ticks.y = element_blank(),
        panel.grid.minor.y = element_blank(),
        panel.grid.major.y = element_blank()) 

13.4.2 Probing moderation of mediation.

We already computed the relevant 95% credible intervals at the end of section 13.3 Relative conditional indirect effects. We could inspect those in a geom_halfeyeh() plot, too.

rcie_tibble_pick_a_point %>%   
  
  ggplot(aes(x = value, y = sexism)) +
  geom_halfeyeh(point_interval = median_qi, .prob = c(0.95, 0.5),
                fill = "grey50", color = "white") +
  labs(x = "Relative Conditional Effect on Liking", y = NULL) +
  theme_black() +
  theme(axis.text.y = element_text(hjust = 0),
        panel.grid.minor.y = element_blank(),
        panel.grid.major.y = element_blank()) +
  facet_wrap(~`indirect effect`)