| ci.c.ancova {MBESS} | R Documentation |
To calculate the confidence interval for an unstandardized contrast in the one-covariate ANCOVA.
ci.c.ancova(Psi, means, error.var.ancova = NULL, c.weights, n, x.bar, SSwithin.x, conf.level = 0.95, ...)
Psi |
the unstandardized contrast of adjusted means |
means |
the vector that contains the adjusted mean of each group |
error.var.ancova |
the error variance obtained from the ANCOVA summary table; i.e., mean square within in the ANCOVA table |
c.weights |
the contrast weights |
n |
either a single number that indicates the sample size per group, or a vector that contains the sample size of each group |
x.bar |
a vector that contains the group means of the covariate |
SSwithin.x |
the sum of squares within groups obtained from the summary table for ANOVA on the covariate |
conf.level |
the desired confidence interval coverage, (i.e., 1 - Type I error rate) |
... |
allows one to potentially include parameter values for inner functions |
lower.limit |
the lower confidence limit of the (unstandardized) ANCOVA contrast |
lower.limit |
the upper confidence limit of the (unstandardized) ANCOVA contrast |
Be sure to use the error varaince and not its square root (i.e., the standard deviation of the errors).
If n receives a single number, that number is considered as the sample size per group. If n
receives a vector, the vector is considered as the sample size of each group.
Be sure to use fractions not the integers to specify c.weights. For exmaple, in an ANCOVA of four groups,
if the user wants to compare the mean of group 1 and 2 with the mean of group 3 and 4, c.weights should
be specified as c(0.5, 0.5, -0.5, -0.5) rather than c(1, 1, -1, -1). Make sure the sum of the contrast weights
are zero.
Keke Lai (Indiana University; LaiK@Indiana.Edu)
Maxwell, S. E., & Delaney, H. D. (2004). Designing experiments and analyzing data: A model comparison perspective. Mahwah, NJ: Erlbaum.
ci.c, ci.sc.ancova
# Maxwell & Delaney (2004, pp. 428-468) offer an example that 30 depressive # individuals are randomly assigned to three groups, 10 in each, and ANCOVA # is performed on the posttest scores using the participants' pretest # scores as the covariate. The means of pretest scores of group 1 to 3 are # 17, 17,7, and 17,4, respectively, and the adjusted means of groups 1 to 3 # are 7.5, 12, and 14, respectively. The error variance in ANCOVA is 29, # and the sum of squares within groups from ANOVA on the covariate is # 313.37. # To obtained the confidence interval for adjusted mean of group 1 versus # group 2: ci.c.ancova(means=c(7.5, 12, 14), error.var.ancova=29, c.weights=c(1, -1, 0), n=10, x.bar=c(17, 17.7, 17.4), SSwithin.x=313.37)