# help interpret interaction effect in linear mixed effect

I am trying to predict response time (transformed in Log) from Group (a factor with two levels: NS and NNS), and Time of Testing (a factor with three levels: Pretest, Posttest, Delayed-test), using linear mixed effect model. The slope coefficient for Group was (-0.455006) suggesting that NNS (the reference level) had larger RT than NS. The slop coefficient for Time of Testing was: -.25 for from Pretest to Posttest, and -.35 from Pretest to Delayedtest. Both coefficient for Group and Time of testing were signifiant. There was also signifiant interaction. The model output is posted below:

Fixed effects:                           Estimate     Pr(>|t|)
(Intercept)                              8.033121  < 0.0000 ***
GroupNS:                                -0.455006    0.00195 ***
Time_of_TestingPosttest:                -0.252902    0.00 ***
Time_of_TestingDelayedtest:             -0.393049    0.000 ***
Item_TypeAr:                             0.027541    0.459570
Item_TypeCong:                          -0.057590    0.202765
Item_TypeEng :                          -0.056005    0.097206 .
scale(Length) :                          0.056470    0.00000189729 ***
scale(Prof_Voc):                        -0.199754    0.000234 ***
scale(AssocFor):                         0.007124    0.624079
scale(PhraseChoice_Norming_NNS):        -0.030128    0.029637 *
scale(For_Comp_Norming_NNS_PERC):       -0.045862    0.019740 *
scale(Back_Comp_Norming_NNS_PERC):      -0.027450    0.088654 .
GroupNS:Time_of_TestingPosttest:         0.125559    0.00***
GroupNS:Time_of_TestingDelayedtest :     0.128348    0.00 ***
GroupNS:Item_TypeAr :                    0.152143    0.00***
GroupNS:Item_TypeCong:                   0.067548    0.00 **
GroupNS:Item_TypeEng:                   -0.125478    0.000 ***
Time_of_TestingPosttest:Item_TypeAr:    -0.118301    0.00 ***
Time_of_TestingDelayedtest:Item_TypeAr: -0.007223    0.80
Time_of_TestingPosttest:Item_TypeCong:  -0.057250    0.03 *
Time_of_TestingDelayedtest:Item_TypeCong 0.015867    0.56
Time_of_TestingPosttest:Item_TypeEng    -0.066561    0.017 *
Time_of_TestingDelayedtest:Item_TypeEng  0.018056    0.51
---
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1


1.) My first question, what does the positive sign in the interaction indicate? How can the positive + or negative - be used to understand the interaction effect relationship?

I had difficulty understating what the slope coefficient mean, so I plotted the graph below.

1. ) However, I still don't see any interaction effect in the sense that Group depends on Time, or vice versa. I would really appreciate your kind assistance in interpreting the interaction coefficient based on the plot
• Can you post your model output in its entirety in your original question? – Isabella Ghement May 14 '19 at 20:16