Hi all,
We are dealing with contradictory results and want to understand how to interpret these. Based on a pre - post assessment of teacher's content knowledge we expected two groups to start out with the same knowledge and the experimental group to gain significantly more knowledge over time than the control group. Indeed a repeated measures ANOVA showed is a significant (p=.002) interaction effect between the groups over time. But a simple ANOVA on the post-test score show no significant difference between the 2 groups. GROUP Mean pre score Mean post score control 18.5 24.5 experimental 17.5 27 We also ran a two-block regression model were we predicted the post test score by a first set of predictors which included group membership and pre test score. In our second set we also entered the interaction as a third predictor. We expected this interaction still to be significant based on the repeated measures ANOVA results. However, according to the regression analysis the interaction was no longer significant. What does this mean and how can we make sense of these contradictory results? Meike Lemmens *********************************************************************** This email may contain confidential material. If you were not an intended recipient, please notify the sender and delete all copies. We may monitor email to and from our network. *********************************************************************** |
Mike,
So it sounds like you ran two different analyses. One was a repeated measures ANOVA. The means for that analysis were as follows. GROUP pre post delta Control 18.5 24.5 6.0 Tx 17.5 27 9.5 You had a significant interaction effect. If you look at the pre-post delta, the Tx group has a bigger increment. But the control-tx difference at post is only 2.5. However, if the control-tx difference at pretest had been 0.0 instead of 1.0, the post group difference would have been 3.5. So, I think that one explanation is that the possibly nonsignificant difference at pretest may be the cause of the nonsiginificant group difference at post. Other things also matter, like within group SD at post. What I think you should do is to use the emmeans subcommand and compare pre-post for control and pre-post for tx. You'll have one of two outcomes: both significant or tx significant but not control. The problem you have is that while both gained, tx gained more. It sounds like your second analysis was the regression equivalent of ANCOVA. While there is no necessary reason for the two analyses to give the same results, I think you made an interpretation error. The interaction that you entered sounds like it must have been group by pretest. If so, then you are testing whether the within group regression slopes are the same between the two groups, which you should fervently hope that they are. You should be looking at the group effect. Gene Maguin |
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