BIO 213 Regression: Interaction between a continuous and categorical variables (effect package)

## Settings for RMarkdown http://yihui.name/knitr/options#chunk_options
opts_chunk$set(comment = "", warning = FALSE, message = FALSE, tidy = FALSE, 
    echo = T, fig.width = 10, fig.height = 8)
options(width = 125, scipen = 5, digits = 5)

setwd("~/statistics/bio213/")

Load data

library(gdata)
lbw <- read.xls("~/statistics/bio213/lbw.xls")
lbw$smoke <- factor(lbw$smoke, levels = 0:1, labels = c("non-smoker","smoker"))

## Same data available online: http://www.umass.edu/statdata/statdata/data/index.html
## Cases 10 and 39 needs fix to make them identical to Dr. Orav's dataset
## lbw2 <- read.xls("http://www.umass.edu/statdata/statdata/data/lowbwt.xls")
## lbw2[c(10,39),"BWT"] <- c(2655,3035)

Fit models

model.smoke.lwt.interaction <- lm(bwt ~ smoke + lwt + smoke:lwt, lbw)
summary(model.smoke.lwt.interaction)

Call:
lm(formula = bwt ~ smoke + lwt + smoke:lwt, data = lbw)

Residuals:
    Min      1Q  Median      3Q     Max 
-2040.3  -456.2    28.6   531.7  1977.7 

Coefficients:
                Estimate Std. Error t value Pr(>|t|)    
(Intercept)      2347.51     312.72    7.51  2.5e-12 ***
smokesmoker        43.90     451.16    0.10    0.923    
lwt                 5.40       2.34    2.31    0.022 *  
smokesmoker:lwt    -2.42       3.39   -0.71    0.476    
---
Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1 

Residual standard error: 709 on 185 degrees of freedom
Multiple R-squared: 0.07,   Adjusted R-squared: 0.055 
F-statistic: 4.64 on 3 and 185 DF,  p-value: 0.00372 

effects package is useful for interaction plot

library(effects)
plot(effect("smoke:lwt", model.smoke.lwt.interaction), multiline = TRUE)