# How to properly compare interacting levels

**URL:** <https://discourse.mc-stan.org/t/how-to-properly-compare-interacting-levels/20457>\
**Category:** brms\
**Tags:** conditional-effects, hypothesis-testing\
**Created:** [January 26, 2021, 8:27pm UTC](https://discourse.mc-stan.org/t/how-to-properly-compare-interacting-levels/20457 "2021-01-26T20:27:18Z")\
**Posts on this page:** 1\
**Showing post:** 5

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**Author:** ![striatum](https://avatars.discourse-cdn.com/v4/letter/s/ac8455/32.png) [@striatum](https://discourse.mc-stan.org/u/striatum)\
**Post date:** [February 1, 2021, 11:48am UTC](https://discourse.mc-stan.org/t/how-to-properly-compare-interacting-levels/20457/5 "2021-02-01T11:48:35Z")

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Thanks so much for this! I am trying to make the model work, but I am getting an error message:

> Error: Factors with more than two levels are not allowed as covariates.  
> So, following my example above, this is my actual model:

```no-highlight
summary.brm.model_zero <- summary(brm.brm.model_zero <- brm(bf(DV ~
    0 + (T + L + S) * C +
    (1|Item) +
    (1|Participant),
        T ~ 0 + type,
        L ~ 0 + GroupsL,
        S ~ 0 + GroupsS,
        C ~ 0 + condition,
        nl=TRUE),
    prior=c(prior(normal(0, 0.5), nlpar=T),
            prior(normal(0, 0.5), nlpar=L),
            prior(normal(0, 0.5), nlpar=S),
            prior(normal(0, 0.5), nlpar=C),
            prior(exponential(1), class=sigma)),
    family=gaussian,
    data = dat,
    chains=4, iter=4000, cores=4,
    control=list(adapt_delta=.95)))

```

So, it interaction the problem here? Or?

Thanks!

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