[R-lang] lmer sliding contrasts

[Ray Becker]

Hi everyone,

I have a problem with my sliding contrasts, and need some help. The means
of the descriptive statistics and t-values from the contrasts are not
adding up. For example, I have run the omnibus lmer and found a significant
3-way interaction (WMGroup x number x response location), t(30) = 2.89; p <
0.05. My DV is response time, my IVs are high- vs. low-working memory, the
grammatical number of the subject noun phrase (singular vs. plural), and
left vs. right response location). After the omnibus lmer, I did sliding
contrasts to test the simple main effects between left vs. right responses
(labeled 'corans' below). First, I code my contrasts into one variable,
'cond':

tme3wcorx$cond <- ifelse(tme3wcorx$WMGroup == 'LOW' & tme3wcorx$sing_plur
== 'p' & tme3wcorx$corans == 8, 100,
ifelse(tme3wcorx$WMGroup == 'LOW' & tme3wcorx$sing_plur == 'p' &
tme3wcorx$corans == 7, 101,
ifelse(tme3wcorx$WMGroup == 'LOW' & tme3wcorx$sing_plur == 's' &
tme3wcorx$corans == 8, 102,
ifelse(tme3wcorx$WMGroup == 'LOW' & tme3wcorx$sing_plur == 's' &
tme3wcorx$corans == 7, 103,
ifelse(tme3wcorx$WMGroup == 'HIGH' & tme3wcorx$sing_plur == 'p' &
tme3wcorx$corans == 8, 104,
ifelse(tme3wcorx$WMGroup == 'HIGH' & tme3wcorx$sing_plur == 'p' &
tme3wcorx$corans == 7, 105,
ifelse(tme3wcorx$WMGroup == 'HIGH' & tme3wcorx$sing_plur == 's' &
tme3wcorx$corans == 8, 106,
ifelse(tme3wcorx$WMGroup == 'HIGH' & tme3wcorx$sing_plur == 's' &
tme3wcorx$corans == 7, 107, NA))))))))

Then I scale and apply the contrast codes:
tme3wcorx$condcent <- scale(tme3wcorx$cond)
tme3wcorx$condcentf <- factor(tme3wcorx$condcent)
c.sliding <- contr.sdif(8)
contrasts(tme3wcorx$condcentf) <- c.sliding

Lastly, I run the lmer with the coded variable now called 'condcentf' with
the other fixed effect 'TenseCent' (verb tense) as a covariate by using a
'+' instead of '*' as I did in the omnibus lmer. I don't think that
matters, but maybe someone might find fault with this approach.
tme3RESPCONTRASTS.lmer1 <- lmer(Resptime ~ condcentf + TenseCent
 + (1|RECORDING_SESSION_LABEL) + (1|Stimnum), data=tme3wcorx)

The two significant differences I find are both for plural-subject noun
phrase sentences.
In the low-WM group, participants are reliably faster when the response is
on the right (M = 3409; SD = 624) compared to the left (M = 3410; SD = 642;
t(30) = -3.20)

In the high-WM group, participants are reliably faster when the response is
on the right (M = 3071; SD = 718) compared to the left (M = 3132; SD = 722;
t(30) = 4.36)

I haven't calculated p-values yet, but consider any t-value over +/-2
significant at the 0.05 level. The problem, as you can see, is not only do
the first set of means only differ by < 1ms, but the difference is in the
same direction as the other set of means. So the t-values should not be
negative and positive, but the same sign.

Please let me know if you can see what I am doing wrong with my sliding
contrasts. Other comments on problems with how I am doing this analysis are
also welcome. Thank you in advance.

Best,
-Ray
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