RM ANOVA Confidence Intervals

Comments

• MaximusLuminis

  May 2023

  For future reference: This is because JASP follows a special procedure for RM ANOVA plots:

  Display error bars: By selecting this option, error bars will be displayed in the plot. The error bars can either represent confidence intervals or standard errors. In order to get accurate confidence intervals and standard errors, the data are normalized by subtracting the appropriate participantʹs mean performance from each observation, and then adding the grand mean score to every observation. The variances of the resulting normalized values in each condition, and thus the size of the bars, no longer depend on the participant effects and are therefore a more accurate representation of the experimental manipulation. See Morey (2008) for a thorough discussion of this procedure.We apply the procedure from Morey (2008), where some normalization is applied, so it is not as straightforward as sd / sqrt(n). See

  Morey, R. D. (2008) Confidence Intervals from Normalized Data: A correction to Cousineau (2005). Tutorial in Quantatitative Methods for Psychology, 4(2), 61-64.

  https://github.com/jasp-stats/jasp-issues/issues/2127
• EJ

  May 2023

  +1
• MaximusLuminis

  July 17 edited July 17

  

  I understand that in version 0.19, Morey's correction has been made optional (with the checkbox "normalize error bars"). However, even with this disabled, the CI bars still deviate from the SPSS graph; they even get a bit smaller and therefore deviate further. Both JASP graphs suggest a notable difference in T3 (no overlap in the CIs), while SPSS does not. I would expect the non-normalized graph from JASP to be identical to the SPSS graph, or am I wrong? What is going on here?

  See the images below:

  JASP normalized:

  

  JASP not normalized:

  

  SPSS:

  

  
  
• EJ

  September 5

  I saw Johnny address this somewhere, but I forgot the location. I'll attend him to this issue.

  EJ
• JohnnyB

  September 5

  Yes, just closed the issue today: https://github.com/jasp-stats/jasp-issues/issues/2127#event-14149289225

  I now made the whole normalization optional, rather than only the Morey bit of the correction (which is also consistent with the option name "normalize error bars"). Hopefully it can still be included in the hot fix.