# Conditional operator in regression

What does the conditional operator (|) do in the linear regression? In the following simulation, I have two sources (s) one of which reports 2 variables (x,z) and the other reports only one variable (x).

library(dplyr)
df1 <- data.frame(x = seq_len(30),  z = c(rep(NA,20), 1:10)) %>%
mutate(
s = ifelse(is.na(z), 1, 2),
y = 2+3*x+5*ifelse(is.na(z), 0, z)
)


I want the model to read z only when it is available and ignore it otherwise. However when i run lm(y~x+z|s, data = df1) I get only intercept term without the coefficients.

# Call:
# lm(formula = y ~ x + z | s, data = df1)
#
# Coefficients:
#   (Intercept)  x + z | sTRUE
#         57.67             NA


I also tried lm(y~x+z*s, data = df1) and lm(y~x+z:s, data = df1) but do not get the result I was expecting.

How is this kind data modeled if not with conditional operator or interaction term? Any pointers would be helpful.

• I assume you know that the "conditional operator" gives the probability distribution for the random variable z when x is a known fixed value. When x is not known the distribution for z could differ from the conditional one. Apr 5 '17 at 21:09
• @MichaelChernick You are correct naturally about standard probability notation, but the question is entirely about R syntax. Apr 5 '17 at 21:37

| means 'group by' and I'm not sure but I don't think it is used in lm at all. In a mixed effects model, it is used to define the random effect. That is, a different random effect is fitted for each value. e.g.

model <- lme(distance ~ age, Orthodont, random = ~ age | Subject)
ranef(model)


where a different random effect for age is fitted for each Subject.

In the example in your question, you are using it in its other meaning of "OR", so you are including a logical in each observation that evaluates to TRUE if (x+z) or s is greater than 0. This is not what you want (I assume). Your model output is telling you that for all cases where x+z | s evaluates to TRUE has an NA additional intercept.

I think you want the interaction between z and s, which is written z:s. Note the different operator. Note that z*s is shorthand for z + s + z:s which is probably why you were getting answers that you weren't expecting as you are including fixed effects for z, s and the interaction.

• Strangely x+z:s also gives me the same coefficients as x+z*s. Apr 6 '17 at 13:10
• Unless you have changed your default na.action option, you are excluding all rows with any NA values (i.e. all of your source 1 in your example). You need to set these to a value (probably zero) to include them. Second, you want s to be a factor df1$s <- factor(df1$s. This will give you an intercept, the coefficient of x the coefficient of z:s1 which is NA given that all z:s1 are zero and the coefficient of z:s2 which is the extra effect of z when source is 2 is in the model. I get 2, 3, NA, 5 for (intercept), x, z:s1, z:s2 in that order (which matches the generator). Apr 6 '17 at 14:51
• Thank you. Coding z to 0 instead of NA gives the expected results. Apr 6 '17 at 15:01