Checking homogeneity of regression slopes

Returns information that can be used to determine if the homogeneity of regression slopes, an assumption for ANCOVA, is met.

HomogeneityOfRegressionSlopes(
  inputted.data,
  dependent.variable,
  independent.variable,
  covariates
)

Arguments

inputted.data	A dataframe
dependent.variable	A string that specifies the column name of the column to use as the dependent variable. Column must be numeric.
independent.variable	A string that specifies the column name of the column to use as the independent variable. Column can be numeric or factor. If it's a factor, then it can only have two levels.
covariates	A vector of strings that specifies the columns names of the columns to be used as covariates. Columns can be numeric or factor. If it's a factor, then it can only have two levels.

Value

A matrix with two rows. The first row specify what the values are in the second row. The second row: The first element is the formula used to evaluate p-value of interaction terms. The remaining elements are the p-values for each interaction term.

Details

To test homogeneity of regression slopes, interaction terms need to be added and the p-value should be assessed. If the p-value is significant for any interaction term, then this means the assumption of homogeneity of regression slopes is not met.

Examples

dependent.col <- c(10.1, 11.3, 12.1, 13.7, 14.2, 1.6, 2.3, 3.2, 4.1, 5.3)
independent.col <- as.factor(c(1, 1, 1, 1, 1, 0, 0, 0, 0, 0))
covariate.one.col <- c(1, 2, 3, 4, 5, 1, 2, 3, 4, 5)
covariate.two.col <- as.factor(c(1, 0, 1, 0, 1, 0, 1, 0, 1, 0))

inputted.data <- data.frame(dependent.col, independent.col, covariate.one.col,
                            covariate.two.col)

results <- HomogeneityOfRegressionSlopes(inputted.data, "dependent.col",
                                         "independent.col",
                                         c("covariate.one.col", "covariate.two.col"))

results
#>      [,1]                                                                                                                                           
#> row1 "Interaction terms Formula (test for homogeneity of slopes)"                                                                                   
#> row2 "dependent.col~independent.col + covariate.one.col + covariate.one.col*independent.col + covariate.two.col + covariate.two.col*independent.col"
#>      [,2]                                       
#> row1 "P-value independent.col:covariate.one.col"
#> row2 "0.109766283989074"                        
#>      [,3]                                       
#> row1 "P-value independent.col:covariate.two.col"
#> row2 "0.367854055970476"