RM ANOVA Confidence Intervals

MaximusLuminis

Hi,

when calculating an RM ANOVA, I encountered a problem. Under "Descriptive Plots" you can set that confidence intervals are displayed. However, these confidence intervals do not match the confidence intervals that are output by SPSS. The confidence intervals in JASP are much smaller. I have attached screenshots of the JASP graph and SPSS graph. Both analyses are based on exactly the same data set. CI's were set to 95% in both cases.

What could be the reason for this?

JASP.png

SPSS.png

MaximusLuminis

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

+1