MBESS (version 4.8.0)

ci.c.ancova: Confidence interval for an (unstandardized) contrast in ANCOVA with one covariate

Description

To calculate the confidence interval for an unstandardized contrast in the one-covariate ANCOVA.

Usage

ci.c.ancova(Psi, adj.means, s.ancova = NULL, c.weights, n,
cov.means, SSwithin.x, conf.level = 0.95, ...)

Arguments

Psi

the unstandardized contrast of adjusted means

adj.means

the vector that contains the adjusted mean of each group on the dependent variable

s.ancova

the standard deviation of the errors from the ANCOVA model (i.e., the square root of the mean square error from ANCOVA)

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

cov.means

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

Value

lower.limit

the lower confidence limit of the (unstandardized) ANCOVA contrast

upper.limit

the upper confidence limit of the (unstandardized) ANCOVA contrast

References

Kelley, K. (2007). Constructing confidence intervals for standardized effect sizes: Theory, application, and implementation. Journal of Statistical Software, 20 (8), 1--24.

Maxwell, S. E., & Delaney, H. D. (2004). Designing experiments and analyzing data: A model comparison perspective. Mahwah, NJ: Erlbaum.

See Also

ci.c, ci.sc.ancova

Examples

Run this code
# NOT RUN {
# 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 obtain the confidence interval for adjusted mean of group 1 versus 
# group 2:
ci.c.ancova(adj.means=c(7.5, 12, 14), s.ancova=sqrt(29), c.weights=c(1, -1, 0), 
n=10, cov.means=c(17, 17.7, 17.4), SSwithin.x=313.37)
# }

Run the code above in your browser using DataCamp Workspace